The Cosmic Distance Scale
The Milky Way
About the Image
Image Credit: The Isaac Newton
Group of Telescopes, La Palma,
and Simon Dye (Cardiff University).
Because we dwell within the Milky Way Galaxy, it is impossible for us to take a picture of its spiral structure from the outside. But we do know that our Milky Way has a spiral nature from observations made from within our Galaxy (though whether or not it is a barred spiral is still being debated). To represent this, the beautiful spiral galaxy Messier 74 was used, as it thought to be a similar galaxy to ours.
Below is a picture of the real Milky Way taken by the satellite COBE. The disk and center region of our Galaxy are readily recognizable. This image makes the Milky Way appear much more galaxy-like and less like the smudge of stars we see stretching across our night sky. It is possible to imagine what our Milky Way might look like looking down on it from outside.
Image Credit: The COBE Project, DIRBE, NASA
Although the light year is a commonly used unit, astronomers prefer a different unit called the parsec (pc). A parsec, equal to 3.26 light years, is defined as the distance at which 1 Astronomical Unit subtends an angle of 1 second of arc (1/3600 of a degree) When we use the parsec for really large distances, we often put a prefix in front of it - like kiloparsecs (kpc), which are equal to 1000 parsecs - or Megaparsecs (Mpc), equal to a million parsecs.
The Milky Way is about 1,000,000,000,000,000,000 km (about 100,000 light years or about 30 kpc) across. The Sun does not lie near the center of our Galaxy. It lies about 8 kpc from the center on what is known as the Sagittarius arm of the Milky Way.
How Do We Calculate Distances of This Magnitude
Parallaxes give us distances to stars up to perhaps a few thousand light years. Beyond that distance, parallaxes are so small than they cannot be measured with contemporary instruments. Astronomers use more indirect methods beyond a few thousand light years.
The methods to measure stellar distances greater than a few thousand light years include:
Proper motions: All stars move across the sky, but only for nearby stars are these motions perceivable, and even then it takes decades or centuries to measure. Statistically, stars move at about the same rate; therefore, the stars that appear to have larger motions are nearer. By measuring the motions of a large number of stars of a given class, we can estimate their average distance from their average motion.
Moving clusters: Clusters of stars, such as the Pleiades and Hyades star clusters, travel together. Analyzing the apparent motion of the cluster can give us the distance to it.
Interstellar lines: The space between stars is not empty, but contains a sparse distribution of gas. Sometimes this leaves absorption lines in the spectrum we observe from stars that lie beyond the interstellar gas. (Absorption lines are colors missing in a continuous spectrum because of their absorption by atoms or ions. The spectrum is the array of colors or wavelengths that is obtained when light is dispersed.) The further a star is, the more absorption will be observed, since the light has passed through more of the interstellar medium.
Inverse-square law: The apparent brightness or magnitude of a star depends both on its intrinsic brightness or luminosity (how bright the star actually is rather than how bright it seems) and its distance from us. The inverse-square law says that the flux from a luminous object decreases as the square of its distance. If we know the luminosity of a star (for instance, we have a measured parallax for one star of the same type and know that others of the same type will have similar luminosities), we can measure its apparent brightness and then solve for its distance. There are several variations on this, many of which are used to measure distances to stars in other galaxies.
Period-luminosity relation: Some stars are regular pulsators, meaning their intensity changes periodically. The physics of their pulsations is such that the period of one oscillation is related to the luminosity of the star. If we measure the period of such a star, we can calculate its luminosity. From this, and its apparent magnitude, we can calculate its distance. The period-luminosity relation was discovered by Henrietta Swan Leavitt in 1908 when she was studying Cepheid Variable stars in the Magellanic Clouds. Cepheids, named after Delta Cephei, the first and most luminous of its class to be identified, make excellent distance indicators, because of their periodicity and extraordinary brightness. Not only can they be found at the far reaches of our Galaxy, they can also be resolved in galaxies outside of our own. The most luminous Cepheids can be used to estimate distances to objects as far as 12,000,000 light years away.
There are complications in using the period-luminosity relationship. First, the relationship itself depends on the chemical composition of the star. Secondly, the absorption of certain wavelengths of light by the interstellar medium can affect the apparent brightness of the star and therefore must be accounted for. Even with these (and other) complications, Cepheid Variables provide an excellent way to measure the relative distances. To convert to absolute distances, we ideally need to measure the distance to a nearby Cepheid with another, more direct, method. There is much debate at present in this area, in particular regarding the Hipparcos measurements of distances to nearby Cepheids. (See the Nearest Stars page for more information on Hipparcos measurements.)
Interestingly, the size of our own Galaxy was debated for a long while. It was not until early in the 20th century that Harlow Shapley used observations of RR Lyrae variable stars to estimate our Galaxy's size. RR Lyrae stars are similar to Cepheid Variables. They have relatively short periods, typically of about a day or less, and all RR Lyrae stars have approximately the same luminosity. Typically, RR Lyrae stars are less luminous than Cepheids, but they are much more common. Globular clusters of stars - swarms of old stars tightly bound together by gravity and orbiting at the outskirts of galaxies, contain many variable stars, including RR Lyraes.
Shapley was able use these to find the distance to the globular clusters that surround our Galaxy. Not only were the globular clusters great distances away, but the Sun did not lie at the center of their distribution, which placed the Sun far from the center of the Galaxy. Shapley's first estimate of the radius of the Milky Way was off by a factor of 2, but he made an important first step in understanding the nature of our Galaxy.
Several more modern methods have been used to map our Galaxy more accurately. The neutral hydrogen gas in our Galaxy emits light at a wavelength of 21 cm; while this light is invisible to our eyes, it is observable to radio telescopes. Other molecules like carbon monoxide also emit radio waves. This is very helpful for mapping the disk portion of our Galaxy.
Why Are These Distances Important To Astronomers?
Distance is a useful tool on the galactic scale. If you can measure the average speed of stars as they move around the Galactic Center and their distance from the Galactic Center, you can make a plot called a "rotation curve". The rotation curve, which describes the motion of the galaxy can be used to determine the amount of mass within a given radius from the center. The predicted rotation curves for many galaxies (in particular, spiral galaxies like the Milky Way) don't match the observed ones, which led to the discovery of dark matter as an explanation for this discrepancy. It is thought that these galaxies consist of a large, round halo of dark matter, with the visible matter concentrated in a disk at its center.
The Voyager spacecraft is traveling away from the Sun at a rate of 17.3 km/s. If Voyager were to travel to the center of our Galaxy, it would take more than 450,000,000 years to travel the 8 kpc. If it could travel at the speed of light, an impossibility due to Special Relativity, it would still take over 26,000 years to arrive!
At 17.3 km/s, it would take Voyager over1,700,000,000 years to traverse the entire length of the Milky Way. Even traveling at the speed of light, it would take nearly a hundred thousand years!