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Ask an Astrophysicist: The Night Sky

Ask an Astrophysicist

Library of Past Questions

The Night Sky

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Library of Past Questions and Answers

    Observing the Skies

What was the object I saw, which brightened and faded quickly? (Submitted October 16, 2000)

The Question

I was in South Carolina last night Oct 15th on a lake looking at the night sky. I was looking straight up and slightly to the north and I saw a medium sized star get really bright and then it disappeared as if it exploded. This event occurred around 7:00pm. Was this a supernova???? If not what was it????? Please let me know.

The Answer

What you saw was probably an "Iridium Flare." This is caused by the satellites of the Iridium mobile phone system, when they happen to line up just right. The Iridium consortium is now bankrupt (apparently nobody wanted to carry around a 10 kg mobile phone when 90% of where most business travelers go is served by cell phones), and the satellites will be deorbited unless a buyer is found. In the meantime, there are web-based programs to predict when you might see such a flare in the future (unfortunately, we don't know of one to tell you if what you saw in the past was a flare).

Anything lasting just a few seconds is much too short a time to be a supernova, or any other such astronomical event. It could have been a meteor that happened to be heading directly toward you so that you didn't see any streak, but your description sounds like an Iridium flare.

One prediction service is at

http://www.heavens-above.com/

You can read about the Iridium satellites and why they flare, as well as look at some photos at

http://www.satobs.org/iridium.html
http://antwrp.gsfc.nasa.gov/apod/ap980402.html

Hope that helps.

-Kevin Boyce and Martin Still,
for "Ask an Astrophysicist"

Question ID: 001016a_2

Why is the sky blue and why is the Sun red at sunrise and sunset? (Submitted June 11, 1997)

The Question

Why is the sky blue and why is the Sun red at sunrise and sunset (taking into account the properties of interstellar dust)?

The Answer

Actually, it is not interstellar dust that is responsible for the color of the sky during the day and the Sun in morning and evening. It is the scattering within the Earth's atmosphere that is responsible.

Visible light is the region of the electromagnetic spectrum with wavelengths from 0.00000035 meters (violet light) to 0.00000075 meters (red light). Light usually travels in a straight line path, but it can be scattered by particles such as the nitrogen molecules in the Earth's atmosphere. This means that portions of the light energy are sent off into different directions. These directions are determined by the size of the wavelength of the light and the size of the particles doing the scattering. For the Earth's atmosphere, the effect is strongest in the shortest wavelengths; the effect is very small at longer wavelengths. The scattering is dominant for the wavelength that we see as blue, and this is why the sky appears blue.

When the Sun is close to the horizon, such as at Sunrise and Sunset, the light must pass through a much longer path in the atmosphere; more of the blue light is scattered out of this direction, leaving only the red light. This is why the Sun appears red at Sunrise and Sunset.

Cheers,
Padi Boyd
for the Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 970611f

Why is space black? (Submitted January 29, 1997)

The Question

Hello, I am 4 years old and I would like to know why space is black. Thank you for taking the time to answer me. (My auntie is typing this for me)

The Answer

This is a very good question!

Do you know why the sky is bright and blue during the day? It's because the Sun is shining on the air, so the air becomes bright.

The Sun is shining in space too, but there is no air for the light to bounce off of in space. That's why space is black.

Best wishes,
Koji Mukai
for Imagine the Universe!

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 970129c

How would you recommend that I go about purchasing my first telescope? (Submitted November 30, 1996)

The Question

I am an absolute beginner in astronomy, and I would like to install a telescope in my home. What would you recommend me to purchase?

The Answer

We cannot give you specific recommendations, partly because you have to decide how much you want to be able to do with your telescope.

To learn about various issues to do with purchasing your first telescope, we recommend the following:

  1. The Dec 1995 issue of Sky & Telescope magazine had an article on "Two Affordable Beginner's Telescopes". If this is not available to you, you may want to look through the back issues of whatever popular astronomy magazines that are in your local library. They are likely to have articles on this subject from time to time, as well as advertisements by those who sell them.
  2. The book "Turn Left at Orion" by Guy Consolmagno and Dan M. Davis (1989, published by Cambridge University Press). This is a guidebook for finding a hundred deep sky objects visible to small aperture (2" - 3") telescopes, and includes information about the Moon, the planets, and a chapter entitled "How to Run a Telescope".
Jesse Allen, Jim Lochner, and Koji Mukai
for Imagine the Universe!

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 961130a2

What magnification do telescopes need to photograph galaxies? (Submitted March 18, 1998)

The Question

I'm in 6th grade and studying the planets and the stars and galaxies and I wanted to know what magnification of telescope it takes to take a picture of other galaxies?

The Answer

Some of the closer galaxies, Andromeda in the Northern Hemisphere and the Magellanic Clouds in the Southern for example, are visible with the naked eye, so photographs can be taken with a 1X telescope, although you would need to have a way to track the galaxy for a long exposure. Far more important than the magnification is the size of the opening, which determines how much light the telescope collects, and the quality of the optics. Although many inexpensive telescopes try and sell selves with magnification claims, a telescope is typically not useful at magnifications more than 1X per mm of opening. So a 60mm refractor is really only useful up to about 60X, although they'll sell them as 200X telescopes.

Thanks for your questions!

Eric Christian
for Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 980318a

How can you measure the amount of light pollution in the sky? (Submitted May 23, 1998)

The Question

I am doing a summer research project on the affects of light pollution on astrophotography. I was wondering if you could relay me any relevant information and hopefully methods for measuring light pollution. I have no way to measure different amounts of light pollution present at any given location and was wondering if it is even possible to measure.

The Answer

I've asked a colleague, George Gliba, who is a very experienced amateur astronomer, who provided me with the following answer.

You may also be interested in the International Dark Sky Association:

http://www.darksky.org/

Best wishes,

Koji Mukai
for Ask an Astrophysicist


The best way to quantatively measure the amount of light pollution is by the determination of the limiting magnitude for stars visible from your area. It is known that, with ideal conditions, the average person should see stars as faint as 6.0 to 6.5 magnitude with the naked-eye. The International Meteor Organization's web site provides some guidelines for determining your limiting magnitude at the web site:

http://www.imo.net/visual/major/observation/lm

This will provide you with different area of the sky maps, whose stars you can count to determine your limiting magnitude, and thus the amount of the light pollution in your night sky.

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 980523a

Why do the stars appear to twinkle while the planets don't? (Submitted December 17, 1998)

The Question

Why do stars twinkle? I have heard that it is because the light is refracted as it passes through the atmosphere, but if that is true, why does light from planets not twinkle?

The Answer

You've heard it right.

Twinkling is closely related to what astronomers call 'seeing' (atmospheric blurring of an image). Both are caused by the turbulent cells in the upper atmosphere: these are little pockets of air that have different density, temperature, humidity etc. than the surrounding air. The density contrast causes refraction, and as different cells move in and out of your line of sight, the image of the star (which is point-like) is seen to move around from one second to the next. This movement is seen as twinkling by the eyes; if you take a photograph over several minutes, as astronomers often do, then the image becomes blurred. The seeing (this blurring) can be as good as ~0.5 second of arc at the best astronomical sites on Earth, while the worst I've ever seen at a professional observatory was about 8 seconds of arc (we gave up observing that night; more typically, 2 seconds of arc would be considered bad by today's professional astronomers). 1 second of arc is 1/3600th of a degree.

So why don't planets twinkle? This is because, even though they may look point-like to naked eyes, they are actually much bigger than the typical seeing. This means that you observe the combination of light which has passed through different atmospheric cells. Thus, the turbulent effects are averaged out, making the planets look steady.

Hope this helps.

Koji Mukai & Maggie Masetti
for "Ask an Astrophysicist"

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Question ID: 981217a

    The Moon and the Planets

Are there special names for the first full moon of each month? (Submitted March 14, 1997)

The Question

March's full moon is often the "Sugaring Moon" and the "Harvest Moon" and the "Blue Moon" are also quite well known, but what are the names of the full moons for each month? A third grade in New York is interested in knowing since we've just done Maple Sugaring.

The Answer

Your message was forwarded to the Ask an Astrophysicist service. Our group of experts was able to dig up quite a few useful references to web pages which can answer your question. Here are some of them.


From a now defunct site (http://www.peconic.net/independent/06199608.htm) on Night Sky (June 19 - July 2, 1996)

The Full Moon of June generally goes by the epithet of the "Rose Moon," or the "Flower Moon," while people in certain areas blessed with berries, as ours is, prefer to call it the "Strawberry Moon." But even in a month with that many Moon monikers to go around, only the first Full Moon gets granted a formal name. Following the Rose/Flower/Strawberry Full Moon of Saturday, June 1, the Sunday June 30 return, like all second comers, will be hailed simply as the "Blue Moon".


The full moon on June 30, 1996 was (barely!) a so-called "Blue Moon," because it occurred as the second Full Moon within the month. In time zones East of Brevard County, Florida, however, this was the first Full Moon of July. Hence, it was called the "Thunder Moon" or "Hay Moon".

All full moons draw attention to themselves by the way they dominate the sky from dusk to dawn, so that each one has acquired at least one special name. Moon names far outnumber muffin varieties, including Green Corn, Thunder, the Sturgeon Moon, and my personal favorite, the Moon of Pairing Reindeer. Crescent, half, and gibbous moons, on the other hand, are merely termed waxing or waning, not named for anything in particular. The full moon of June generally goes by the pet name Rose Moon, while slightly less discriminating observers dub it the Flower Moon, and romantics in certain fruited areas prefer to gaze at the Strawberry Moon. But even in a month with that many moon monikers, only the first full moon gets a formal name. Following the Rose/Flower/Strawberry full moon of Saturday, June 1, the Sunday, June 30, return, like all second comers, will be hailed simply as the Blue Moon.

(Note, however, this meaning of "Blue Moon" is based on a mistake: http://SkyandTelescope.com/observing/objects/moon/article_377_1.asp.)


From a now defunct site (http://www.noble.mass.edu/ref/fullmoon.htm)

Native American Names

January - Wolf Moon
February - Snow Moon, Hunger Moon, Opening Buds Moon
March - Maple Sugar Moon, Worm Moon
April - Frog Moon, Pink Moon, Planter's Moon
May - Flower Moon, Budding Moon
June - Strawberry Moon
July - Blood Moon, Buck Moon
August - Moon of the Green Corn, Sturgeon Moon
September - Harvest Moon
October - Hunter's Moon, Moon of Falling Leaves
November - Beaver Moon
December - Cold Moon


From http://www.fabandpp.org/cotm/moons.htm

January - Old Moon, Wolf Moon
February - Snow Moon
March - Sap Moon, Worm Moon, Crow Moon, Crust Moon
April - Grass Moon, Pink Moon, Moon of the Red Grass Appearing
May - Milk Moon, Flower Moon
June - Rose Moon, Strawberry Moon
July - Thunder Moon, Buck Moon
August - Green Corn Moon, Corn Moon, Sturgeon Moon
September - Fruit Moon, Harvest Moon
October - Harvest Moon, Hunter's Moon
November - Frost Moon, Beaver Moon
December - Long Night Moon, Cold Moon


I hope this helps to answer your question!

Tim Kallman, Karen Smale, and Pat Tyler
for the Ask an Astrophysicist team

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 970314a

Why does the Moon looks much larger near the horizon (at moonset) than in the middle of the sky? (Submitted December 01, 1996)

The Question

I am looking for the right person to explain to me why the Moon looks much larger near the horizon (at Moonset) than in the middle of the sky.

The Answer

This is a psychological effect --- when the Moon is high on the sky, there is nothing to compare the size with except for the whole sky, and it seems small. When it is near the horizon, your mind compares it with the detailed features on the horizon, and the Moon appears to be bigger.

You can perform an experiment yourself to confirm this. See, for example, http://nfo.edu/moonsize.html

Koji Mukai
for Imagine the Universe!

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 961201a

Why does the Moon looks white in daytime while it is yellow at night? (Submitted October 21, 1997)

The Question

I am three years old. Since I cannot type, I am having my Dad write you. I asked this question to my Dad, who is an astronomer, but he could not answer. I guess he is silly. My question is simple: 'Why does the Moon looks white in daytime while it is yellow at night?'.

The Answer

There are a couple of things which might cause this.

Color vision is not very well understood. It is known that the eye and brain try to adjust the colors you see to correct for the color of the light shining on it. That is why grass looks green even when you see it under the red light of sunset. When you look at the Moon during the day, your eye sees the blue background of the sky, and your brain thinks that the light is blue, and (incorrectly) figures out what color the Moon must actually be to look the way it does under blue light. When you look at it at night, the brain has more trouble since it has no way of guessing what color the light is. This might account for the apparent color difference.

Another possibility, which is certainly part of it, is that the color difference is due to skylight. When you look at the Moon during the day, you see the moonlight, plus all the blue sunlight which is scattered by the atmosphere between the Moon and you. At night, the atmosphere doesn't have any sunlight to scatter.

David Palmer
for Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 971021d

Why did the moon turn red during the last lunar eclipse? (Submitted November 05, 1997)

The Question

I would like to know why the moon turned red during a recent lunar eclipse?

The Answer

When you saw the moon turn red, it was because the light that was hitting the moon, from the Sun, had to go through the Earth's atmosphere. The atmosphere scatters blue light more than red light (why the sky is blue) and so what comes out the other side is red (why sunsets are red). This reddish light bounces off the moon, comes back to Earth and goes into your eyes.

Cheers,

Jonathan Keohane for Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 971105a

What causes a ring to appear around the moon? (Submitted January 02, 1997)

The Question

Last night I looked at the Moon. The Moon was full and there was a huge ring around it. I have seen this type of thing before and I don't know what it is. I call it a Moon ring. I was wondering if you could tell me what it is called and how it happens. I think it has something to do with clouds. I am not a scientist but I do have an interest in astronomy. Please write me back.

The Answer

While our expertise is in high-energy astrophysics, we can point you in the right direction for the answer to your question. You are correct in thinking that it is an atmospheric effect. The ring around the Moon is caused by the reflection of Moonlight (which of course is reflected sunlight) from ice crystals in the upper atmosphere. The shape of the ice crystals results in a focusing of the light into a ring. Since the ice crystals typically have the same shape, namely a hexagonal shape, the Moon ring is always the same size.

Here is the title and author of a good reference book to physical phenomenon seen in our atmosphere (geared for HS senior or college freshman level):

Minnaert, M., The Nature of Light and Color in the Open Air, Dover Publications, New York, 1954.

Sincerely,
Andy Ptak and Michael Arida
for the Ask an Astrophysicist team

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 970102b

What are Sundogs and Moondogs? (Submitted February 07, 1997)

The Question

I was wondering if you knew the names of the Sun dogs and moon dogs?

The Answer

Sundogs and moondogs are the names given to rainbow-like patches sometimes seen on either side of the Sun or the Moon. They are not real dogs and do not have names as far as I know. They are similar to the ring sometimes seen around the Moon.

Sundogs and the ring around the Moon are caused by the focused reflection from ice crystals in the upper atmosphere. Since the ice crystals are hexagons, the distance the sundogs are from the Sun and the radius of the ring around the Moon are always the same size in the sky (22 Degrees).

Many cultures hold the ring around the Moon as a sign of good luck, since it associated with good rainfall and healthy crops.

You can see a picture of Sundogs on the WWW at:

http://www.astropix.com/HTML/G_SUN/SUNDOGS.HTM

Thanks for your interest,

Jonathan Keohane (with help from much of the Ask an Astrophysicist team)

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 970207e

Are there planet charts? (Submitted August 09, 1997)

The Question

It would be helpful to see charts of the stars and planets. Are there charts available on the Internet showing maps of the sky indicating the location of the planets each month? There is a large planet now appearing nightly. Is it Venus?

The Answer

When Venus is visible, it always appears as a very bright object fairly close to the horizon, either in the west just after sunset or in the east before sunrise.

Sky and Telescope offers an excellent monthly online column with highlights of the northern hemisphere night sky and a printable star chart at http://SkyandTelescope.com/observing/.

Happy Star Gazing !!

Jim Lochner, Leonard Garcia, Brian Hewitt, and Gail Rohrbach
for Ask an Astrophysicist

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Question ID: 970809

Would you know a place that I can find out the positions in degrees of the planets on a given day? (Submitted July 10, 1997)

The Question

I live on Long Island and I was wondering if by any chance you would know a place that I can find out the Positions in degrees of the Planets on a given day. I need this for a reference for a program.

The Answer

There are shareware and freeware programs available on the Internet that will calculate this for you. You'll find links to lots of astronomy software for different platforms at

http://www.physics.sfasu.edu/astro/software.html

Damian Audley
for the Ask an Astrophysicist Team

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Question ID: 970710b

How do you calculate the positions of planets from any point on Earth at any given time? (Submitted October 27, 1997)

The Question

I'm trying to find information on how to calculate the positions of planets from any point on Earth at any time on earth.

I know that software exists to do just this. However, I would like to write my own and compare it to the others.

The Answer

Calculating the motions of the planets is a simple exercise in integrating the equations of motion. In principle, this can be done with reference only to an introductory physics text if you don't already know how to write down the equations. In reality, there are already lots of books and programs which can solve this problem, or tell you how. A good place to start is:

http://aa.quae.nl/en/reken/hemelpositie.html

I hope this helps!

Tim Kallman
for Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

Question ID: 971027d

    Stars and Constellations

Where can I get a chart of the Southern Skies? (Submitted May 22, 1997)

The Question

I am going to Macchi Picchu (Peru) and want to study & take with me a star chart of the Southern Hemisphere. Where can I get hold of one?

The Answer

Most good star chart books will include charts for the entire sky, both Northern and Southern Hemispheres. A few things to look for are:

1. A book that opens to a page and stays there. When you are out looking at stars, life is much easier if you can rely on your star chart book to stay open at the page you want when you put it down to look with binoculars

2. Large page format. There are a lot of nice books of star charts that small enough to fit in your pocket, but they often resist staying open to the page you want and also tend to have smaller print and charts which are harder to read in the dark.

3. If you are planning to do deep sky observing now or in the future (i.e. you plan to look with a telescope for galaxies, hunt for comets, etc), look for charts that include dimmer stars. Star brightness is usually measured in "stellar magnitudes " with bright stars having small magnitudes (e.g 1.0 for the bright star Vega) and dimmer stars having larger magnitudes. Generally a good dark sky will allow you to see stars down to perhaps 6th magnitude. But there are only a few thousand stars above 6th magnitude, so a telescope view is more likely than not to not include a 6th magnitude guide star.

I have heard general good reviews of Norton's Star Atlas and Peterson's Field Guide to the Stars and planets.

If you are just looking with your eyes or binoculars, you might find a planisphere more useful. These are charts mounted on a disk with a finder that allow you to adjust the charts to local time so you can see what it up at that particular time. These are specific to a latitude, and the more equatorial location of Peru may be a slight problem. Sky Publishing Corporation (the publishers of Sky & Telescope and CCD Astronomy magazines) make a Southern Hemisphere planisphere. You might want to try visiting their WWW site at

http://skyandtelescope.com/

If there is a local telescope store in your area, they can probably answer many more of your questions, point out interesting objects in the southern sky to look for, offer more specific advice, and show you a wider range of products. A good camera store may also have star charts and such, though they are less likely to have staff who are avid amateur astronomers. If these fail, you might also try one of the national telescope suppliers, such as Meade, Celestron, or Orion. They all have toll-free contact numbers, and a quick search on the WWW also turned up URLs for Orion and Meade at

http://www.oriontel.com/

http://www.meade.com/

We hope this helps. Enjoy your southern skies!

Leonard Garcia, Jesse Allen and others
for "Ask an Astrophysicist "

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Question ID: 970522b

How do the stars move in the sky? (Submitted May 18, 1997)

The Question

Why do stars seem to travel across the sky at night differently than the moon moves? Do stars rise and set like the Sun and Moon? The stars seem to stay in the same place while the earth moves in circles, is this how it works? I appreciate any explanations about the change in star positions.

The Answer

When we look at the night sky we see stars in different locations depending on the time of night. We also see different groups of stars depending on the time of year. The reason we see stars move during the night is because the Earth spins. Because the Earth spins, we see the stars rise and set, just as the Sun and Moon do. In addition to rising and setting, however, the Sun and the Moon have an additional motion across the sky.

Try this experiment. Go out one night when the Moon is visible and try to find some stars that appear close to the Moon. You might want to draw a picture showing the Moon and the location of these stars. Try to go out the next night at the same time and compare your drawing to what you see. The stars you drew should be in about the same spot as the night before but the Moon will have moved. What happened? The stars are in the same spot because the Earth spun around once. The reason why the Moon isn't quite where you saw it the night before is because the Moon is orbiting the Earth. It takes about 27 days for the Moon to orbit the Earth one time. What happens when you go looking for the same stars a month later at the same time? You will probably notice they are in a slightly different part of the sky then they were before. Why? This is due to the Earth's motion around the Sun. The Earth orbits the Sun in one year and in one year you will see many different groups of stars in the sky. These groups are the constellations. Because the Earth orbits the Sun you will see different constellations, for example, at night in the winter months than in the summer.

So, the motion of the stars we see over one night is due to the Earth spinning. The motion you might see over several weeks or months is due to the Earth orbiting the Sun.

Leonard Garcia and Jim Lochner
for Ask an Astrophysicist

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Question ID: 970518

What are the three brightest stars in the Summer Sky? (Submitted April 04, 1998)

The Question

What are the three brightest stars in the summer sky?

The Answer

What the three brightest stars in the summer sky are depends on your location and the time of night.

However, the three prominent bright stars high overhead in the early Summer evening in the Northern temperate zones are called 'The Summer Triangle'. These are Altair, Deneb, and Vega. These form a bright triangle, with the Southernmost star (farthest from the other two) being Altair, the brightest one (to the West) being Vega, and the one to the East being Deneb.

David Palmer
for Ask an Astrophysicist

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Question ID: 980404a

How many stars are there, named and un-named, known to exist? (Submitted January 15, 1997)

The Question

How many stars are there, named and un-named, known to exist ?

The Answer

This is a very good question! There are too many stars for scientists to actually count one-by-one, so other methods of estimating the total number of stars are used. We believe that there are on the order of 1021 stars in our Universe. If you write that number out, it looks like this: 1,000,000,000,000,000,000,000. This is a lot of stars!

Sincerely,
Laura Whitlock
for the Ask an Astrophysicist team

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Question ID: 970115

Are the stars we see at night all from our own galaxy? How many stars are in our galaxy? (Submitted February 02, 1998)

The Question

Are the stars we see at night all from our own galaxy? How many stars are in our galaxy?

The Answer

We can only see a few thousand stars at most with our unaided eyes. These are a mixture of stars which are nearby, and bright stars which are further away; but they are only a tiny fraction of the 100,000,000,000 stars in our own galaxy. We can't see stars in other galaxies without powerful telescopes. In fact the entire brightest neighboring galaxy (M31, the Andromeda galaxy), which contains more stars than our own, is only as bright as an average star visible to the unaided eye.

Paul Butterworth
for the Ask an Astrophysicist team

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Question ID: 980202g

How bright is Antares? (Submitted February 21, 1997)

The Question

I am 34 years old, graduated from Belhaven College in Jackson Mississippi in 1987 with a Business Major. I am from Santa Cruz Bolivia and four years ago we founded an Insurance Brokers with the name of ANTARES BROKERS because it is the brightest star of our galaxy. In this matter I would appreciate if you could help me by sending me information about this star and if it is possible a poster or a picture of it. Please let me know if I have to pay for this information in order to send the check.

The Answer

First the bad news! Antares, although very luminous, is not the brightest star in the Galaxy. As you may know, Antares is only the 15th brightest star as viewed from the Earth (and excluding the Sun). If we could move all the stars in the galaxy to the same distance from us, so that we could compare their true brightnesses, Antares would be impressive - but there would be many even brighter stars. Antares may be the brightest star within about 1000 light years of us, but go a little beyond that and Deneb, Betelgeuse and Rigel are all about 10 times brighter. Currently the brightest known star in the Galaxy is Cygnus OB2 number 12, which is about 200 times brighter than Antares.

Now the good news! Antares is a pretty spectacular object - to be in the most luminous hundreds of stars in a galaxy of billions is no mean achievement. It is a red supergiant, which means that it has consumed all its hydrogen and is still shining only because it has begun to fuse together increasingly heavy elements. When all the fuseable elements have been consumed, Antares will collapse and explode in a supernova - at which time its brightness will rival that of the rest of our galaxy put together. Antares is also close in the sky to some pretty nebulae, so you should be able to find just the picture you want. I know of some beautiful images obtained by David Malin at the Anglo-Australian Telescope, at least one of which is available as a slide. (Go to http://www.syz.com/images/ and enter 'Antares' as your search string). You should contact the Royal Observatory, Edinburgh (by going to their site at http://www.roe.ac.uk ) for details.

With best wishes,

Paul Butterworth and Mike Arida
for Imagine the Universe!

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Question ID: 970221b

Can you tell me about Fomalhaut? (Submitted February 28, 1998)

The Question

I would like some information about the star Fomalhaut. All I need is an address to a good information page or something like that.

The Answer

Fomalhaut is the 17th brightest star in the sky and is in the southern hemisphere constellation Piscis Austrinus (The Southern Fish), so it is also called Alpha Piscis Austrini.

(see: http://www.astro.wisc.edu/~dolan/constellations/hr/8728.html)

Piscis Austrinus is not a well known constellation because the rest of the constellation is rather dim.

According to Peterson's Field Guide, Fomalhaut is "a bright white star whose brilliance in intensified by the comparative darkness of the starless background. Alpha Piscis Austrini is also called Fomalhaut, a name derived from the Arabic for "mouth of the fish." At magnitude 1.16, Fomalhaut is the 18th brightest star in the sky. To northern-hemisphere observers it is visible in autumn, low above the horizon, in an empty region of the southern sky." It is mere 23 light-years away. Its luminosity is 14 times that of our sun. Like the Sun, it does not have a gravitationally bound partner.

Try looking this star up in other observational astronomy books!

Good luck,

Maggie Masetti, Karen Smale & Jonathan Keohane
for Ask an Astrophysicist

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Question ID: 980228a

Could you give me the names of all the stars in the constellation Pegasus? What about other constellations? (Submitted May 17, 1997)

The Question

Could you give me the names of all the stars in Pegasus (or any other constellation)?

The Answer

The stars have historically gone by different naming schemes. The Greeks, the Arabs, the Egyptians and other cultures often gave names to the stars, often based on a myth. When astronomers started collecting stars into catalogues they gave them more systematic designations. A common one still in use is a designation in order of brightness using Greek letters and the name of the constellation. Hence, alpha Peg is the brightest star in Pegasus, beta Peg is the second brightest etc. For some constellations, one runs out of Greek letters. Another system simply numbers the stars in a constellation according to their right ascension (a type of "longitude" used in the sky).

The best resource for the names of the stars is a book entitled "Star Names: Their Lore and Meaning" by Richard H. Allen (Dover Publications, 1963). It gives many of the historical names which the Arabs, the Greeks, the Egyptians and other cultures gave to the stars. Hence, there may not be a single name for a star. However, taking the first entries, here are the names for some of the bright stars in Pegasus:

alpha Peg = Markab
beta Peg = Scheat
gamma Peg = Algenib
epsilon Peg = Enif
Zeta Peg = Homam
eta Peg = Matar
Theta Peg = Baham
Lambda Peg = Sad el Barr
tau Peg = Sad Al Naamah

There is a nice web page where you'll find information on the stars in any constellation at

http://www.astro.wisc.edu/~dolan/constellations/

For more detailed information about particular stars (e.g. a star's distance, magnitude, temperature, etc.), see

http://www.astro.utoronto.ca/~garrison/oh.html

Jim Lochner
for Ask an Astrophysicist

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Question ID: 970517

Was the present North Star Closer to the pole in 44 B.C.? (Submitted February 03, 1998)

The Question

I'm at the "watches NOVA and sometimes reads Scientific American" sort of level.

Shakespeare has Brutus say "I am as constant as the northern star." Given the precession of the Earth's axis, was Polaris close to the celestial north pole in 44BC? Was another bright star closer? Or is this an anachronism on Shakespeare's part?

The Answer

The Earth precesses much like a top, the projected rotation axis of the Earth to trace out two small circles (North and South) on the sky, of radius 23 degrees, taking about 26,000 years to complete one precession.

We do know that about 5,000 years ago, around the time the Great Pyramid at Giza and Stonehenge were being constructed, the Earth's north pole pointed near the star Thuban, in Draco. 2000 years ago, the north celestial pole will have been some 12 degrees from Polaris and the nearest bright star will have been Kochab (in the tail of Ursa Minor) 8.5 degrees away.

(In about 12,000 years, the Earth's axis will be directed towards the star Vega, which is in the constellation Lyra.)

I think it's safe to say that Shakespeare didn't know about precession!

Paul Butterworth and Maggie Masetti
for the Ask an Astrophysicist team

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Question ID: 980203a

Where are the Pleiades located in the sky and the myths that go along with them? (Submitted January 27, 1998)

The Question

Where are the Pleiades located in the sky and the myths that go along with their formation?

The Answer

The Pleiades are a group of bright white stars that are in the constellation of Taurus. You can find them by following the belt of Orion through a reddish star called Aldebaran. To help find Orion and other constellations see:

http://www.astro.wisc.edu/~dolan/constellations/constellations.html

and

Sky & Telescope gives maps of the sky for each month:

http://skyandtelescope.com/

As for myths, I know that in Greek mythology the Pleiades are the seven sisters who are pursued by Orion. Also in Hindu mythology they are the wives of the seven sages. Either because of their bright white color or some cross-talk between different cultures they have acquired a female set of myths.

Stepping outside the classical mythology, there are loads of stories about the Pleiades! For instance, the following is from the Monache Indians in central California. A group of women who love onions more than their husbands enter the sky to become the Pleiades. Their husbands pursue them and become the constellation Taurus. One source for Native American stories about the Pleiades is the book, "They Dance in the Sky" by Jean Monroe and Ray Williamson. Once again it is interesting that these stars are connected with women.

Enjoy,

Jeff Silvis and Jim Lochner
For Ask an Astrophysicist

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Question ID: 980127b

What determines the starting point of the stellar magnitude scale? (Submitted March 02, 1998)

The Question

How did scientists determine the starting point for the magnitude scale? I am in eighth grade and have a fair background in astronomy. I understand that brighter stars may have negative numbers on this scale, but how did someone determine "zero" on this scale?

The Answer

The magnitude system goes back to the second century B.C. when the Greek astronomer Hipparchus divided stars into six classes. The number one stood for the brightest and six was the faintest. In 1856, Norman Pogson replaced this system with one based on mathematics, but he tried to match the old system as closely as he could. He used the formula:

m = -2.5 * log (F/Fstand)

Now if you don't understand this formula, I'll help you through it. First I'll define the terms:

m - the magnitude
F - is the flux from our star
Fstand - is the flux from a standard star

By the way, flux is basically the amount of energy arriving at earth from the star. Now the tricky part of this is the "log". That is a function that tells you home many powers of ten are in a number. For example:

log(1000) = 3
log(100) = 2
log(10) = 1
log(1) = 0
log(.1) = -1
log(.01) = -2
log(.001) = -3

So with a little knowledge of the log function, we can tell where the zero point of the magnitude system is. When the flux of the star is the same as the standard, then we have:

m = -2.5 log(1)
m = -2.5 * 0
m = 0

So when our star has the same flux as the standard star its magnitude is zero. Now a negative magnitude would be when a star is brighter than the standard star. Most likely this will happen when we calculate the absolute magnitude. This is when we correct for distance. We pretend that all the stars were moved to some standard distance and then we determine what the magnitude would be. Sometimes when the star is "closer", it is brighter than the standard and the absolute magnitude is negative.

In modern use, the star Vega defines magnitude 0.0.

Hope this helps,

Jeff Silvis
For Ask an Astrophysicist

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Question ID: 980302a

How did the stars get their names? (Submitted April 06, 1998)

The Question

I have recently been doing a project about Space in one of my classes. One of the questions we are to answer is 'how did the stars get their names?' Can you help?

The Answer

The stars have a variety of names. There are the common names - Rigel, Sirius, Betelgeuse, Vega, etc - and there are the "scientific" names - Beta Orionis, Alpha Canis Majoris, Alpha Orionis, Alpha Lyrae. The stars may also have names given to them by different catalogues, ex HD 34085, HD 48915, HD 39801, HD 172167.

The common names generally originate from ancient times. Most of the names we use for the stars are Arabic, Egyptian or Greek in origin. These names are often tied to mythology. A good source for the origins of the names of the stars is the book, "Star Names: Their Lore and Meaning" by Richard H. Allen (Dover Publications, 1963). On the web, you might try http://www.astro.wisc.edu/~dolan/constellations/ (although I don't think it gives detailed information).

The scheme of designating stars with Greek letters and the name of the constellation was first used by J. Bayer in 1603.

Jim Lochner
for Ask an Astrophysicist

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Question ID: 980406c

What is the best way to learn how to find the constellations? (Submitted February 01, 1997)

The Question

I am trying to learn about the constellations and how to pick out the different ones by star gazing. Can you help me do that in this site?

The Answer

For learning about the constellations on the web, we recommend:

http://www.astro.wisc.edu/~dolan/constellations/

However, it may be more useful to buy a planisphere (star wheel), the current issue of "Sky & Telescope", "Astronomy"or a similar magazine, or a book on stars and constellations. You can take any one of these outside with you when you go star gazing, and compare what you see on the sky with what is on the chart.

Hope this helps,

Koji Mukai
for Imagine the Universe!

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Question ID: 970201

Which constellation is directly behind the Sun when the Sun rises? (Submitted December 04, 1996)

The Question

I would like to know which constellation is directly behind the Sun when the Sun rises ?

The Answer

The constellation which the Sun is in varies from month to month as the Sun moves in the sky. In December, the Sun is in Sagittarius, while in June it's in Taurus. The magazines Sky & Telescope and Astronomy Magazine each have a plot of the position of the Sun on the sky for that month's issue. You should be able to use that to answer your question for whatever date you are interested in.

Cheers,
Steve Snowden and Jim Lochner
for Imagine the Universe!

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Question ID: 961204a

Where can you find Sagittarius in the sky on winter and summer nights? (Submitted December 01, 1996)

The Question

Where can you find Sagittarius in the sky on winter and summer nights? And how do I know when I've found it?

The Answer

Sagittarius is a summer constellation --- the best time of the year is August to see it at 9pm. Sagittarius is also a Southern constellation: if you're in North America, it can be seen roughly towards the south and will not rise very high on the sky.

You can find out the shape the stars in Sagittarius make

  1. by checking it at http://www.astro.wisc.edu/~dolan/constellations/
  2. by buying a planisphere (star wheel)
  3. by looking up a magazine like "Sky & Telescope" or "Astronomy".
  4. or by buying a book on stars and constellations.
Some people have likened this shape to a tea-pot, which might help you remember once you've seen a picture. If you're not familiar with the summer sky, you might want to start by locating the neighboring constellation, Scorpius, which I find is easier to spot, then look to its left.

Hope this helps,

Koji Mukai
for Imagine the Universe!

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Question ID: 961201b

What would the constellations look like if they were viewed from someplace other than Earth? (Submitted May 29, 1997)

The Question

I would like to know if you traveled to the outermost planet in our solar system if the constellations would appear to change their shapes. What about if you traveled to the next nearest star? Or, if you traveled to the center of the Galaxy, ignoring interstellar dust, would you still see the constellations found in the Earth's night sky?

The Answer

What a great question! Thanks for asking and being interested.

The short answer to your question is that you would definitely see the same constellations from Pluto and definitely not from the center of the Milky Way. As far as the nearest star, they would probably be somewhat similar but not completely.

The pattern of stars that we call the constellations we see because of both where we are and where the stars are. Also, all the stars in a particular constellation are not the same distance away.

For example, take the three brightest stars in Orion (Rigel, Betelgeuse and Bellatrix): they are 250, 150 and 210 parsecs away respectively (1 parsec = 1pc = 3.26 Light Years). Now, imagine that you lived on a planet near Betelgeuse, then Rigel and Bellatrix would appear on opposite sides of your night sky; when one is up the other is down. (As an interesting aside, you would not be able to see our Sun from there because it would be too dim).

On the other hand, if you lived on the nearest star (alpha Centauri), which is only 1.3 pc away from us in a different direction, you would not see much difference. This is simply because 1.3 pc is small compared to the 150 pc Bellatrix is away. There are, however, some very close (but much dimmer) stars in Orion (e.g. epsilon Eridani and o2 Eridani) that are only 3.3 and 4.9 pc away. These would appear to move a great deal to someone on alpha Centauri. Also, our Sun would be a bright star in the sky to someone on alpha Centauri, and some other bright stars would appear in a different place (like Sirius, the brightest star, which is only 2.7 pc away).

Thank you again for you thoughtful question.

Jonathan Keohane and Tess Jaffe
for Ask an Astrophysicist

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Question ID: 970529a

    Other Night-time Objects

What is a shooting star? (Submitted November 03, 1997)

The Question

I would like to know if shooting stars are real, and what makes them move? Thank you for your answer.

The Answer

Shooting stars come from meteoroids, small pieces of material left over from the formation of the solar system, which are entering the earth's atmosphere and burning up as they do. You can find out more on our web pages:

http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level1/meteoroids.html

and

http://imagine.gsfc.nasa.gov/ask_astro/earth.html#970409b

I hope this helps!

Tim Kallman
for the Ask an Astrophysicist Team

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Question ID: 971103a

What are the dates of annual meteor showers? (Submitted January 13, 1997)

The Question

Hello, I am writing a novel and I would like to get information on meteor showers that happen yearly.

The Answer

Here are the dates of the prominent meteor showers that occur every year. I Hope it is of help to you.

Shower When Seen
Quadrantids Jan. 1-5
Lyrids April 16-25
Eta Aquarids April 19 - May 28
Delta Aquarids July 8 - Aug. 19
Perseids July 17 - Aug. 24
Orionids Oct. 2 - Nov. 7
Leonids Nov. 14-21
Geminids Dec. 7 - 17

Good luck with your novel.

Regards,
Laura Whitlock
for Imagine the Universe!

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Question ID: 970113c

How did the Milky Way get its name? (Submitted February 05, 1997)

The Question

How did the Milky Way gets its name ?

The Answer

On a clear night, out in the country away from the city lights, you will see a bright, but diffuse, band through the sky. It will make a complete arc overhead (actually it appears as a great circle on the sky with the earth as the center).

Next time you are out in the country, look at this band of light and think about how it looks. This was named by the Greeks as: "Galaxies Kuklos" or The Milky Circle. The Romans changed the name to "Via Lactea" or The Milky Road or as we now call it "The Milky Way."

However, it was not until the the middle of the 18th century that people first came up with the idea that The Milky Way was actually a galaxy of stars. And it was not until the 19th and 20th centuries that scientists understood that The Milky Way is just one of many such galaxies in the universe.

If you want a good book to read on the history of astronomy, try "Coming of Age in the Milky Way" by Timothy Ferris, c1988, Anchor Books, ISBN 0-385-26326-0

Thank you for being interested.

Sincerely,
Jonathan Keohane
(for the Ask an Astrophysicist Team)

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Question ID: 970205

What is the tale of the two lovers kept apart by the Milky Way? (Submitted May 11, 1997)

The Question

What is the tale of the two lovers that were kept from each other by the Milky Way? They were stars

The Answer

The story of two lovers kept apart by the Milky Way is told in a number of Asian countries. It originates in China, but I'm also aware of versions in Japan and Vietnam. The stars are Vega (in the constellation Lyra) and Altair (in the constellation Aquila).

I'm most familiar with the Vietnamese story, which is entitled "The Weaver Fairy and the Buffalo Boy". You can find it in a book entitled "Sky Legends of Vietnam" by Lynette Dyer Voung. Here is a brief summary of the story:

The Weaver Fairy lives in the sky. She weaves silken robes for the other fairies. She falls in love with the Buffalo Boy, who lives on the earth herding buffalo and playing his bamboo flute all day. The Weaver Fairy lives for a time on earth with the Buffalo Boy, but she is forced by her father, the Jade Emperor, to return to the sky. She pleads that the Buffalo Boy be allowed to live in the sky with them. Her father agrees, but only if he tends the herds of buffalo that dwell in the sky, and she returns to weaving silk robes for the rest of the fairies. The Buffalo Boy comes to live in the sky, but he and the Weaver Fairy are so happy to be together again that they soon neglect their duties. The Jade Emperor punishes them by putting the Weaver Fairy and her loom on the east bank of the Silver River, and the Buffalo Boy and the buffalo on the west bank. They beg for one more chance. The Jade Emperor allows them to meet once a year, on the seventh day of the seventh month. Every year they do so, and they are so happy that their tears of joy fall on the earth.

The story explains the apparent coming together of these two stars during the summer months. It also explains (from some things I didn't include in the summary) the disappearance of crows in the summer, the summer rains, and the appearance of rainbows.

I hope you'll look up the full story in Lynette Dyer Vuong's book, or in other collections of Chinese or Japanese star stories.

Jim Lochner
for Ask an Astrophysicist

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Question ID: 970511c

When is the best time to view a comet? (Submitted March 20, 1997)

The Question

What's the best time to view comets?

The Answer

The best time to view a comet depends on its individual orbit and its brightness. The recent comet, Hale-Bopp, had an orbit that took it higher into the evening sky and lower in the morning so that in April, the evening view became more and more impressive.

Scientists believe that Hale-Bopp might have also been on one of its early trips into the inner solar system. It last visited the inner solar system about 4,000 years ago. Because it has come close to the Sun only a few times before, it shed much more dust and gases than comets which have come through the inner solar system many times. Comet Halley, for instance, has been through the inner solar system many many times and so it tends to shed much less material.

In addition to the Sky & Telescope site, you may want to check out: The Hale-Bopp Page.

For more general information about comets, we suggest

http://ssd.jpl.nasa.gov/?comets

Comets have been very much in the news the last few years: we hope you find observing them to be rewarding!

Jesse Allen
for Imagine the Universe!

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Question ID: 970320a

Can you see satellites with the naked eye? (Submitted April 14, 1997)

The Question

Perhaps you can settle a heated debate between my wife and I and thereby preserve domestic tranquility.

Can you visually locate and observe SATELLITES from a rural area (Wisconsin) with the unaided, naked eye?

The Answer

We don't get called on for domestic disputes too often, but here goes...

You can see satellites pass overhead from a dark site, such as rural Wisconsin. What you see is sunlight reflected from the satellite: since the satellite is higher up in the sky, it may not be in the Earth's shadow even though you are (otherwise it would not be night!). You can tell that an object you see is a satellite rather than a plane or star because it will move steadily across the night sky over the period of a few minutes, often "disappearing" as it crosses into the Earth's shadow. As the night progresses, you will see satellites "disappear" closer to the horizon until after several hours after sunset, even satellites in low Earth orbit (about 200-800 miles above the Earth's surface) will be in the Earth's shadow. Satellites in geosynchronous orbits are high enough (about 22,000 miles up) that they will almost always be sunlight, but they are too far away for them to be visible.

The space station Mir occasionally passes overhead in the evening sky in the United States after sunset and can be seen. You can check for information on Mir passes with astronomy magazines. There may be similar information available (though I have not seen it) for the space shuttle during their flights and/or for other large satellites such as the Hubble Space Telescope.

We hope this settles your domestic issue amicably...

Jesse Allen
for "Ask an Astrophysicist"

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Question ID: 970414f

Where can I find a list of all the visible satellites? (Submitted May 05, 1997)

The Question

I'd like to find a list of visible satellites in the evening sky. I understand that there may be many, but I'm certain that several obvious ones would be noticeable.

The Answer

From any point on Earth, there are many visible satellites that pass over each night. Which satellites are visible, and when and where they appear, has to be calculated knowing the date and the location of the observer.

There are programs available to do these calculations, such as OrbiTrack for the Macintosh. Go to your favorite shareware and public domain site (e.g. www.shareware.com) and search for 'satellite'.

You will also need a list of current satellite 'elements' which describe the orbits of each satellite. These are provided by T. S. Kelso at http://celestrak.com/NORAD/elements/ The file tle.new has the most recent elements available, while the data for the Mir space station is in mir.tle . (tle stands for 'two line elements', because all the data for a satellite is written on two lines.)

With a program and a fresh data file (they lose accuracy after a few weeks) you can get a list of visible satellite passes in your area.

David Palmer
for Ask an Astrophysicist

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Question ID: 970505h



 

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