AST1002 HW 6

The following statements describe ways in which the raisin analogy might apply to the real universe. Which statements are correct?

-The raisins stay roughly the same size as the cake expands, just as galaxies stay roughly the same size as the universe expands -The average distance increases with time both between raisins in the cake and between galaxies in the universe -An observer at any raisin sees more distant raisins moving away faster, just as an observer in any galaxy sees more distant galaxies moving away faster

Properties associated with irregular galaxies

-They exhibit vigorous star-forming activity -Some show evidence for prior collision or close encounter with another galaxy -They are typically smaller than spiral galaxies (Irregular galaxies are typically smaller than spiral or normal ellipticals, of irregular form, and without spiral arms. Irr I galaxies are often characterized by lots of gas, dust, and associated star formation. Irr II galaxies have forms that indicate prior collisions or close encounters with other galaxies. Irregular galaxies are one of the most common types of galaxies)

What is the approximate distance of our Sun from the center of the Galaxy?

8kpc (8,500pc or 28,000LY)

Irregular galaxy (Irr)

Asymmetric, often with gas, dust, and young stars (Irregular galaxies lack the symmetric structure seen in the spiral and elliptical galaxies. Some irregular galaxies are rich in gas and dust and are the sites of vigorous star formation)

Spiral galaxy (S)

Central bulge, flattened disk, spiral arms, gas, dust, young stars (Spiral and barred-spiral galaxies have central bulges typically containing old stars along with flattened disks, which harbor both old and new stars as well as clouds of gas and dust)

Know how to sort the galaxy types according to their level of star-forming activity

Elliptical - little star-forming activity Spiral, barred-spiral, and irregular - significant star-forming activity

Barred-spiral galaxy (SB)

Elongated central structure, flattened disk, spiral arms, gas and dust, young stars (Spiral and barred-spiral galaxies have central bulges typically containing old stars along with flattened disks, which harbor both old and new stars as well as clouds of gas and dust)

Elliptical galaxy (E)

Round, no disk, very little gas and dust, only old stars (Astronomers have since learned that elliptical galaxies contain mostly old, red stars and very little gas or dust)

Each of the four basic galaxy types is readily recognizable by its apparent shape. Name the different types of galaxies

Spiral (S), Barred-spiral (SB), Elliptical (E), Irregular (Irr) (The Hubble galaxy classification system is based on the four basic galaxy types—elliptical, spiral, barred-spiral, and irregular. As depicted in the Hubble tuning fork diagram, the different galaxy types form a sequence of decreasing central concentration, symmetry, and smoothness of structure. Edwin Hubble originally thought that galaxies evolved over cosmic time from the simple elliptical structures to the more ragged irregulars. As astronomers have learned more about these various types, they have concluded that the Hubble Sequence does not correspond to an evolutionary progression. Astronomers have since determined that unless galaxies interact with each other, most galaxies retain their original structures)

____, ______, and ______ galaxies contain star-forming clouds along with blue regions of recently formed hot stars. By contrast, _______ galaxies no longer have these features

Spiral / barred-spiral / some irregular Elliptical

As we progress in the Hubble classification sequence of spiral galaxies from type Sa to Sb to Sc, how do the physical characteristics change?

The central bulges become smaller The spiral arms become less tightly wound

The Tully-Fisher relation uses the close correlation between the rotational speed of a spiral galaxy and its luminosity as a distance indicator. Why does this correlation exist?

The greater the mass of a galaxy, the faster it spins, and the more mass there is to give off light (Mass controls both the rotation rate and the luminosity of the luminous matter)

Which statement best describes the distribution of halo stars and globular clusters and the location of our Sun, relative to the galactic disk of the Milky Way Galaxy?

The halo stars and globular clusters are distributed in a roughly spherical region surrounding and centered on the disk; the Sun is located roughly halfway out from the center in the plane of the disk. (Harlow Shapley's insight was that the globular clusters map out the approximate extent of the stars of the Galaxy, which have since been found in a spherical region approximately 30 kpc across. The Sun resides in a spiral arm of the disk, about 8 kpc from the Galaxy's center.)

By looking at the graph of Hubble's Law, what can we say about the galaxies that have the lowest speeds?

They are moving away from Earth and are closer to Earth than galaxies with high speeds (This fact also follows from Hubble's law. Note that this applies to galaxies outside the Local Group. Within the Local Group, some galaxies move toward us, attracted by the gravity of the Milky Way)

Since the time of Hubble, astronomers have learned that the ______ color observed in some galaxies is the result of recent star formation (This interpretation has been confirmed by multiwavelength observations that have revealed the presence of star-forming gas clouds in galaxies hosting newly formed O-and B-type stars)

blue

Stars orbiting in the ______ near the galaxy's center can have orbits highly inclined to the galactic plane

bulge

The circular but relatively flat portion of the galaxy is the

disk

Where did the Big Bang occur?

everywhere in the universe at once, because the entire universe (including space itself) was a point

A tightly packed group of a few hundred thousand very old stars is a

globular cluster

The first portion of the galaxy to form was the

halo

The graph of Hubble's Law shows that galaxies with high speeds as measured from Earth are

moving away from Earth and are farther from Earth than galaxies with lower speeds (This is the essence of Hubble's law: More distant galaxies are moving away from us faster)

The fact that the energy output of active galactic nuclei can vary significantly over a short time indicates that the central object emitting the energy is which of the following?

much less than a parsec across (Instabilities in the accretion disk around the supermassive black hole create energy fluctuations. Because of these fluctuations, astronomers know that the accretion disk is much less than a parsec across)

Our Milky Way galaxy is a ______ galaxy

spiral

A _______ stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust

spiral arm

Multiwavelength observations of the gas and dust in different types of galaxies show a strong correlation between

the amounts of cool gas and dust and the levels of star-forming activity as inferred from the visual appearances of the galaxies These smoother- and redder-appearing galaxies ceased forming stars a long time ago

Suppose that galaxy B is twice as far from Earth as galaxy A. Hubble's law predicts that galaxy B will be moving away from Earth with approximately _____ the velocity of galaxy A

twice (As the graph shows, Hubble's law (the straight-line fit to the data) predicts that a galaxy's recession velocity is proportional to its distance from Earth. Hubble's law therefore predicts that a galaxy twice as far away moves at twice the speed, a galaxy three times as far away moves at three times the speed, and so on)

The age of the universe is related to the slope of the graph of Hubble's law, and current data put the age of the universe at about 14 billion years. Suppose that future observations showed that the slope of Hubble's law on the graph is actually steeper than that shown. In that case, the age of the universe would be _________ than 14 billion years because the universe is expanding ______ than current data suggest

younger / more rapidly (A more steeply sloped line for Hubble's law would indicate faster speeds for galaxies at all distances, which would mean a faster rate of expansion. And a faster rate of expansion means it has taken less time since the Big Bang for the universe to reach its present size. In other words, a steeper slope (which means a larger value of Hubble's constant) means a younger and more rapidly expanding universe)

Imagine that when we looked out into the universe we found that the light from all galaxies was blueshifted (rather than redshifted) and that the light from the most distant galaxies was blueshifted by the greatest amount. Which statement best describes what we would conclude about the motions of galaxies in this case?

All are moving toward Earth, with distant galaxies moving faster than nearby galaxies (In this hypothetical case, all the galaxies would be rushing toward Earth, with the more distant galaxies approaching at the fastest pace. This would tell us we lived in a collapsing universe rather than an expanding one)

Why are infrared and radio telescopes the instruments of choice for studying the galactic center?

Dust in the plane of the Milky Way obscures observations at other wavelengths.