Norma, scuitum-crux, Sagittarius, Orion, Perseus, cygruus

geologist John Michell realized that it would be theoretically possible for gravity to be so overwhelmingly strong that nothing -- not even light traveling at186,000 miles an hour could escape. John later published his theory for a mathematician and philosopher Pierre Simon Laplace to study.

The nearest black hole (Cygnus X-1) is at least 10,000 Light-years away. What is the nearest black hole?

Black holes can have a mass up to 50 billion suns. The lowest mass can be 3 billion suns. What is the mass of a black hole?

Black holes has matter being compressed into a massive density. Sometimes it can collect gas and have it spiral around or go in one side and out the other. Black holes can be as hot as supernovas then cool. There is also no atmosphere around a black hole because everything around gets sucked in. What do black holes do?

the first evidence for dark matter was discovered in the 1930sin the early 1980s astronomers became convinced that most of the mass holding galaxies and clusters of galaxies together is invisible. It wasn't until the beginning of the 21st century that the "Double Dark" standard cosmological model was accepted How was dark matter found?

cold dark matter -- non-atomic matter different from what makes up the planets, stars, and us -- plus dark energy together making up 95% of the cosmic density

Surprisingly, dark matter appears to comprise the bulk of the entire universe

Clues revealed by the recently sharpened view of the Hubble Space Telescope have allowed astronomers to map the location of invisible "dark matter“ in unprecedented detail in two very young galaxy clusters

Dark matter is matter that is inferred from gravitational effects on visible matter and background radiation but, is undetectable By emitted or scattered electromagnetic radiation

At the beginning of the 20th century, astronomers believed that our Milky Way was “the universe”— except two mavericks who insisted some objects were far beyond the Milky Way and were themselves star systems like ours. One young astronomer using their work two decades later set astronomy on its ear. How were galaxies found?

Galaxies are systems of millions or billions of stars, together with gas and dust, held together by gravitational attraction Each has a diameter of nearly 1 billion light-years, about a 10,000 times wider than the Milky Way Galaxy The galaxy is only about 100,000 light-years wide and 1,000 light-years thick

Approximately 3% of galaxies observed cannot be classified as either ellipsoidal or spirals. These galaxies have little symmetry in their structure and are termed irregular galaxies. An example is Sextons A, shown in the image on the right.

Elliptical galaxies lack the cool gas and luminous stars at large galactic radii which have been used so successfully to measure the masses of spiral galaxies. However, from early Einstein observatory X-ray observations, SAO scientists Forman, Jones and Tucker found that luminous elliptical galaxies have a corona of hot (10e7 degrees K) gas. Through X-ray observations which measure the gas density and temperature of this hot interstellar medium, one can determine the mass and distribution of the dark matter required to gravitationally bind the hot gas to the galaxy. Hot coronae have been found around nearly all normal luminous ellipticals as well as around some, more distant, radio galaxies.

An irregular galaxy is a galaxies that does not have a regular shape, like a spiral or an elliptical galaxies. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bugle nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull.

The most common type of galaxy is called a "spiral galaxy.“ Not surprisingly, spiral galaxies look like spirals,\ with long arms winding toward a bright bulge at the center. But be careful - if you looked at a spiral galaxy from the side, you could mistake its shape for a circle, and so you'd have to use other criteria to learn it was a spiral.