C. Black Holes
Black holes are objects so dense that not even light can escape
their gravity and, since nothing can travel faster than light, nothing can
escape from inside a black hole. Nevertheless, there is now a great deal
of observational evidence for the existence of two types of black holes:
those with masses of a typical star (4-15 times the mass of our Sun), and
those with masses of a typical galaxy. This evidence comes not from seeing
the black holes directly, but by observing the behavior of stars and other
material near them!
Galaxy-mass black holes are found in Active Galactic Nuclei (AGN). They
are thought to have the mass of about 10 to 100 billion Suns! The mass of
one of these supermassive black holes has recently been measured using
radio astronomy. X-ray observations of iron in the accretion disks may
actually be showing the effects of massive black holes as well.
The Electromagnetic Spectrum as a Probe of the
All objects in our Universe emit, reflect, and absorb
electromagnetic radiation in their own distinctive ways. The way an object
does this provides it special characteristics which scientists can use to
probe an objectís composition, temperature, density, age, motion,
distance, and other chemical and physical characteristics. Astronomers can
time events (for instance, recording exactly when a binary star system is
eclipsed and for how long), can obtain the energy distribution of a source
(by passing its electromagnetic radiation through a prism or grating to
break it into component colors), or can record the appearance of a source
(such as taking an image of the source). These three methods are by no
means exclusive of each other, but each reveals different aspects of a
source and each method gives the astronomer slightly different
While the night sky has always served as a source of wonder and
mystery, it has only been in the past few decades that we have had the
tools to look at the Universe over the entire electromagnetic (EM)
spectrum and see it in all of its glory. Once we were able to use
space-based instruments to examine infrared, ultraviolet, X-ray, and gamma-ray emissions, we found objects that were otherwise invisible to us (e.g.,
black holes and neutron stars). A "view from space" is critical
since radiation in these ranges cannot penetrate the Earth's atmosphere.
Many objects in the heavens "light up" with wavelengths too
short or too long for the human eye to see, and most objects can only be
fully understood by combining observations of behavior and appearance in
different regions of the EM spectrum.
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