Tutorial on Galaxies

Galaxies are swarms of stars isolated from other stars and held together by their own gravity. They come in a variety of sizes. Small galaxies may contain only a few million stars but large ones may contain 1012 (a trillion) stars.

Within a galaxy, each star follows its own orbit, held there by the gravitational attraction of the other stars in the system.

In addition to their stars, most galaxies also contain clouds of gas and dust. The dust in such clouds absorbs light and you can see such clouds silhouetted against the background of stars in many galaxy pictures.

1.Do we live in a galaxy?

2. What is the name of our galaxy?
A)...The Local Group.
B)... Orion.
C)...The Milky Way.
D)... The Zodiac.
E)...NGC 8742

Up until the early 1900's, many astronomers thought the Milky Way was the only galaxy. They knew of dim, fuzzy patches of light in deep space, but most astronomers thought them to be satellites of the Milky Way.

3. What made astronomers believe these fuzzy patches were other galaxies?
A)...They could see them spin.
B)...They could see them move in orbit around us.
C)...They discovered their distances were millions of light years.
D)...They could see they looked exactly like the Milky Way.
E)...Aliens told them.

4. How did astronomers first measure the distance to galaxies?
A)...From their parallax.
B)... From the brightness of variable stars in them.
C)...From the speed of their spinning motion.
D)...From the speed of their spinning motion.
E)... From laser beams reflected off them.

Measurements of nearby galaxies showed that they are typically about a million or so light years apart. Today, astronomers have seen galaxies as far away as 12 billion light years.

Studies of galaxies soon showed that most were moving away from the Milky Way.

5. How did astronomers discover that galaxies are mostly moving away from the Milky Way?
A)...They could be seen to be growing smaller.
B)...Some would disappear behind remote dust clouds.
C)... Radar signals.
D)... Spectra of their light showed that their absorption lines were shifted to the red.
E)... Spectra of their light showed that their absorption lines were shifted to the blue.

6. What else was soon discovered about their motion?
A)...Farther galaxies were found to recede faster than closer galaxies.
B)... Farther galaxies were found to recede slower than closer galaxies.
C)...They all looked like they were orbiting around the Milky Way.
D)...The bigger galaxies were moving away fastest.
E)...The galaxies spinning fastest were moving away fastest.

7. What is the law called that describes how the speed of a galaxy depends on its distance?
A)...Shapley's relation.
B)... The Twining-Rayburn effect.
C)...Zwicky's relation
D)...Hubble's Law.
E)... Schwarzschild's measure.

A galaxy's distance, D, is related to its speed, V, by V = HD, where H is called the Hubble constant. If V is measured in km/sec and D is measured in millions of parsecs (megaparsecs, abbreviated mpc), then H is about 65 (km/sec)/mpc.

Note: 1 megaparsec is about 3 million light years.

Astronomers are still unsure about the value of H. It is very difficult to measure accurately.

The above relation, called Hubble's Law, allows us to find the distance to a galaxy if we know its recession speed, or vice-versa.

For example, suppose a galaxy is 5 mpc away from us. How fast is it moving according to the Hubble law?

Given that V = HD and that D = 5 mpc, then V = 65 (km/sec)/mpc x 5mpc = 65(km/sec)x5=325 km/sec

8. If a galaxy is 15 mpc away from us, what is its speed according to the Hubble Law?
A)... 15 mpc/sec.
B)...15 km/sec.
C)... 975 km/sec.
D)... 97.5 km/sec.
E)...4.3 km/sec.

The Hubble Law is extremely important to astronomers because it allows them to find a galaxy's distance if they know its recession speed.

9. How can the recession speed be found?
A)...From the shift in the spectrum lines.
B)... From the brightness of the galaxy.
C)...From the galaxy's mass.
D)... From the mass-velocity law.
E)...From Kepler's third law.

10. Suppose a galaxy has a recession velocity of 1300 km/sec. How far away is it?
A)... 13 mpc.
B)... 130 mpc.
C)...2 mpc.
D)...20 mpc.
E)...200 mpc.

Galaxies come in a wide variety of shapes, but most have one of three main shapes: spirals, ellipticals, and irregulars.

11. What type of galaxy is the Milky Way ?

Spiral galaxies themselves divide into two main groups: ordinary and barred spirals. In ordinary spirals, the arms emerge from a small round-ish core. In barred spirals, the arms emerge from the end of an elongated "bar." Many astronomers believe the Milky Way is actually a barred spiral.

12. How do astronomers measure the mass of a galaxy?
A)...From its Doppler shift.
B)...From its color.
C)...From the motions of its stars and gas.
D)... From its shape.
E)...From its radius.

13. Which of the following laws is used to find a galaxy's mass?
A)...Hubble's Law.
B)...Wien's Law.
C)... Schmidt's Law.
D)...The Stefan-Boltzmann Law.
E)... A modified form of Kepler's Third Law.

A galaxy's mass affects the orbital motion of its stars and gas because its mass determines its gravity and the gravity in turn controls the motion of the stars and gas. Therefore, astronomers can work backwards from the motions to get the galaxy's mass.

The law they use states that for a star orbiting with a period P in years at a distance a in AU from the center of a galaxy, the galaxy's mass, M(in solar masses) is given by

M= a3/P2

Note that in using this expression, we normally have to express the star's distance from the galaxy's center in AU, NOT parsecs or light years. If we have the distance in parsecs, it is easy to convert to AU by simply multiplying by 2x105, the number of AU in a parsec.

For example, suppose a star is orbiting a galaxy 10,000 pc from the center with a period of 150 million years. What is the galaxy's mass?

To use the modified form of Kepler's third law, we first express a in AU. If a=10,000 pc, then a is 10,000x2x105 = 2x109 AU.

Now plugging into our expression, we find

M= a3/P2=(2x109)3/(150 million) 2= 23x1027/(1.5x108)2=8x10 27/2.25x1016= 3.6x1027- 16=3.6x1011 Solar Masses.

14. What is the mass of a galaxy in which a star 8,000 pc from the center orbits in 3x108 years?
A)... 4.55x108 solar masses.
B)... 4.55x1010 solar masses.
C)...2.4x108 solar masses.
D)...2.4x1010 solar masses.
E)...5.6x105 solar masses.

Measurements of a galaxy's mass such as we have described above often disagree strongly with the mass deduced from the light of the visible stars and gas.

15. his discrepancy has led astronomers to deduce that__________
A)...Kepler's law fails at large distances.
B)...galaxies contain unseen or "dark matter."
C)...distant galaxies are made of different material than nearby ones.
D)... they need more powerful telescopes to make more accurate measures.
E)...distant galaxies radiate most of their energy at x-ray wavelengths.

16. What have astronomers proposed dark matter might be?
A)...Black holes.
B)... White dwarfs.
C)...Dim, low mass stars.
D)... As yet undetected subatomic particles.
E)...All of the above.

Astronomers understand only approximately how galaxies form and what determines their shape (spiral, elliptical, or irregular). The basic picture of galaxy formation is similar to a scaled-up version of star formation. That is, galaxies form when gravity causes a huge gas cloud to collapse not long after the birth of the Universe. As the cloud shrank in size and its density increased, stars began to form in it.

Because the cloud was so huge and its gas so tenuous, it took tens of millions of years to collapse. In such a long time, some of the stars that formed in the cloud evolved to the end of their life and exploded as supernovas. Although the cloud originally contained only hydrogen and helium, the supernova blasts mixed the heavy elements made in these stars into the gas, enriching it with the elements from which Earth and we ourselves eventually formed.

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