Ast 105 Chapter 15: Our Galaxy

protogalactic cloud

The beginning of our galaxy, of hydrogen and helium. It has been modeled to successfully explain the basic differences between halo and disk stars, but it may be a bit too simplistic.

gas-star-gas cycle

The galactic recycling process which begins with the star's formation, the gas emitted from dying stars

shock fronts

Abrupt, high gas pressure "walls" that move faster than the speed at which sound waves can travel through interstellar space.

Where do stars tend to form in our galaxy?

Active star forming regions, marked by the presence of hot, massive stars and ionization nebulae, are found mostly in spiral arms. The spiral arms represent regions where a spiral density wave has compressed gas clouds to make star formation more likely.

ionization nebulae

Also called emission nebulae or H II regions. Colorful, wispy blobs of glowing gas often near hot stars. These glow because electrons in their atoms are raised to high energy levels or ionized when they absorb ultraviolet photons from the hot stars

interstellar medium

Clouds of interstellar gas and dust that fill the galactic disk, which obscures our view of most of the galaxy when we observe with visible light.

spiral density waves

Disturbances in the spiral pattern of star formation that propagate through the gaseous disk of a spiral galaxy like the Milky Way.

atomic hydrogen gas

Gas that is cool enough that hydrogen atoms remain neutral rather than being ionized. After the gas in bubbles cool, it becomes part of the atomic hydrogen gas in the galaxy.

How did our galaxy form?

Halo stars probably formed in several different protogalactic clouds of hydrogen and helium gas. Gravity pulled those clouds together to form a single larger one. The collapse of this cloud continued until it formed a spinning disk around the galactic center. Stars have formed continuously in the disk since that time, but stars no longer form in the halo.

bubble

Hot, ionized gas ejected from supernovae or powerful stellar winds that sweeps up interstellar material. They are common in the disk of the galaxy, but not always easy to detect.

What is the evidence for a black hole at our galaxy's center?

Orbits of stars near the center of our galaxy indicate that it contains a black hole about 4 million times as massive as the sun. The black hole appears to be powering a bright source of radio emission known as Sagittarius A*.

spiral galaxy

Our galaxy, the Milky Way, is a spiral galaxy so named because of the "spiral arms".

cosmic rays

Rays produced by supernovae that may affect life on Earth by causing genetic mutations in living organisms. They are made of electrons, protons, and atomic nuclei that zip through interstellar space at close to the speed of light.

disk population

Sometimes called Population I, consists of stars that follow the orderly orbital patterns of the disk. Includes both young stars and old stars, all of which have heavy element proportions near 2% like that of our sun.

halo population

Sometimes called Population II, consists of stars that orbit the center of the galaxy with many different inclinations, so that they cross through the disk during their orbits. Stars in this population are all stars that are old and low in mass. Can have heavy element proportions as low as 0.02%, meaning that heavy elements are about 100 times rarer in these stars than in the sun.

How is gas recycled in our galaxy?

Stars are born from the gravitational collapse of gas clumps in molecular clouds. Massive stars explode as supernovae when they die, creating hot bubbles in the interstellar medium that contain the new elements made by these stars. This gas cools and mixes with the interstellar medium, forming what we call atomic hydrogen gas. The gas can then cool further to make molecular clouds in which new stars form, completing the star gas star cycle.

How do stars orbit in our galaxy?

Stars in the disk all orbit the galactic center in about the same plane and in the same direction. Halo stars also orbit the center of the galaxy, but their orbits are randomly inclined to the disk of the galaxy. Some bulge stars orbit like halo stars, while others orbit more like disk stars. Orbital motions of stars allow us to determine the distribution of mass in our galaxy.

What does our galaxy look like?

The Milky Way Galaxy is a spiral galaxy consisting of a thin disk about 100,000 light years in diameter with a central bulge and a spherical halo that surrounds the disk. The disk contains an interstellar medium of gas and dust, while the halo contains only a small amount of hot gas and virtually no cold gas.

What do halo stars tell us about our galaxy's history?

The halo generally contains only old, low mass stars that have much smaller proportion of heavy elements than stars in the disk. Halo stars therefore must have formed early in the galaxy's history, before the gas settled into a disk.

interstellar dust grains

Tiny, solid flecks of carbon and silicon materials that resemble particles of smoke and form in the winds of red giant stars.