1. Chapter 15, Galaxies
Galaxies come in different size and shape. In the previous chapter, we talked about how galaxies provide an environment for the stars to be born and die, and enrich the heavy element content of the galaxy. In this chapter, we will talk about
Galaxy Classification
Location of Galaxies in the Universe
Galaxy Evolution
Quasars and AGN (Active Galactic Nuclei)…

2. Spiral Galaxies NGC (New General Catalog) 4594…
NGC 1300 – barred spiral
NGC 4594 – Sombrero Galaxy. Large bulge, small disk…

3. Elliptical and Irregular
M87 – Elliptical Galaxy in Virgo
Large Magellanic Cloud, irregular galaxy

4. Galaxy Classification
Galaxies come in different size and shape…
Spiral Galaxies
Barred Spiral : Spiral galaxy with a bar.
Lenticular galaxy: Spiral galaxy without spiral arms.
Formed by gas clouds with large initial angular momentum.
Elliptical Galaxies
Similar to the bulge of the spiral galaxies.
Formed by gas clouds with small initial angular momentum.
Formed by high density clouds—more efficient cooling, faster star formation, exhausting the gas supply before the galaxy has time to collapse into the disk.
Formed by collision and merging of galaxies.
Irregular Galaxies
Usually more distant galaxies.
Starburst galaxies
Galaxies with high star forming rate. Merging galaxies?
Quasars, Active Galactic Nuclei
Distant objects. Extremely luminous—could be 1,000 times the luminosity of the Milky Way.
Supermassive Black holes?

5. Galaxy Facts Grouping…
Spiral galaxies are usually found in loosely associated small group (tens) of galaxies
Ellipticals are commonly found in large cluster of galaxies containing hundreds or thousands of galaxies extending over tens of millions of light-years.
Size…
Most of the large galaxies are spiral galaxies…
While some of the largest galaxies are giant elliptical galaxies, the most common type of galaxy in the universe is small elliptical galaxy.
Very small ellipticals (or dwarf spheroidals, less than a billion stars) are often found near large spirals – our Milky Way galaxy has 10 or more nearby…

6. Where are the Galaxies?
Where are the galaxies located? Are they located within the Milky Way, or are they much further away from us than the stars?
Before the 1920s, there were no reliable methods of measuring the distance to the galaxies. Many people believed that the galaxies were located within the Milky Way…
How do we measure the distance of objects far away in the universe, much farther than the distance that can be measured by stellar parallax?
Measurement of distance farther than the reach of stellar parallax rely on our ability to find objects with known luminosity…
In 1924, Edwin Hubble determined the distance to the Andromeda galaxy using Cepheid variables, thus proving that the galaxies are located far beyond the stars in the Milky Way galaxy.

7. Measuring Cosmological Distance
The principle method of measuring astronomical distance is the
distance-luminosity relation
However, the apparent brightness B is the only thing we can measure accurately in most of the cases.
If we know the distance D, we can determine the luminosity L.
If we know the luminosity L, then we can determine the distance D.
Need to find Standard Candles — astronomical objects with known luminosity.
Main sequence stars.
Cepheid variables.
White dwarf supernovae.
Galaxies (using Tully-Fisher Law).

8. The Cosmological Distance Ladder
Methods of Measuring Distance and their useful range…
Radar ranging – D < 10-4 light-years
Parallax – D < 103 light-years
Standard Candles
Main sequence stars – D < 105 light-years
Cepheid variables – D < 107 light-years
White dwarf supernovae – D < 1010 light-years
Hubble’s Law – 1010 ly and beyond…

9. Calibrating the Cosmic Tape Measure
We rely heavily on the standard candles for the measurement of the cosmological distance. How do we make sure that these standard candles are truly standard?
Use independent measurements to check the luminosity of the standard candle.
For example, we can use parallax measurements of the distance to main sequence stars to check measurements of distance using main-sequence fitting. If we do this for a few of them, then we can verify the assumption that the main sequence stars are good standard candle. However, this method works only for stars that are relatively close by.
The standard candles we had verified in close range can now be used (by extrapolation) to measure the distance to more remote objects. This new distance measure then allows us to calibrate the next standard candle.
For example we used the distance measured by observation of Cepheid variables to check the assumption of the constancy of the white dwarf supernovae.
Keep going…

10. Main Sequence Fitting
Main sequence stars with the same color should have the same luminosity. So, if we compare the (pseudo) H-R diagram of a star cluster with unknown distance (using their apparent brightness instead of luminosity) to that of a group of main sequence stars with known distance, then we can determine the distance to this new cluster.
For example…
Hyades (in constellation Taurus) is a open cluster about 150 light years away. It’s distance is close enough to be measured by stellar parallax.
Comparing the H-R Diagrams of Hyades with that of Pleiades, we can determine that the distance of Pleiades should be 2.75 times farther than Hyades…or, 150*2.75 = 410 light years.
New parallax measurement of Hyades by Hipparcos (ESO Space Interferometry mission) yielded a distance of 438 light years
The two parallel lines shows that the apparent brightness of all the different type of main sequence stars are decreased by a factor of 7.5

11. Cepheid Variables
Cepheid variable stars are population I (metal-rich) yellow giant stars with periodic luminosity variation….
Their periods range from a few days to over 100 days,
Their luminosities range from 1000 to 30,000 L⊙,
The high luminosity makes it possible to identify them from a large distance…
Their luminosity and period are strongly correlated. Therefore, we can determine their luminosity by simply measuring their periods!
The luminosity of Cepheid variabls are strongly correlated to their periodicity…

12. Cepheid Variables in M100
The period-luminosity relation of Cepheid variables were discovered Henrrietta Leavitt in Edwin Hubble identified Cepheid variables in Andromeda galaxy (about 2.5 million light-years away) in 1924, and used the luminosity-distance relation to demonstrate that galaxies are much farther than the stars….
There are well over 1,000 Cepheid variables known today…for example, the Polaries!
In 1994, Hubble Space Telescope observed a Cepheid variable in the face-on spiral galaxy M100 in Virgo Cluster located at a distance of 56 million light-years…this is the most distant distant Cepheid observed so far….

13. Tully-Fisher Relation
Although this is not discussed in our text book, the luminosity of the spiral galaxies are related to their rotational speed,. This was discovered by B. Tully (of UH/IfA) and J.R. Fisher in Therefore, the luminosity of the spiral galaxies can be determined simply by measuring their rotational speed…
Spiral galaxies are good standard candles also!
The slope of the luminosity-rotation rate curve is different for different type of spiral galaxies…

14. White Dwarf Supernova
Every time the hydrogen shell is ignited, the mass of the white dwarf may increase (or decrease, we don’t know for sure yet).
The mass of the white dwarf may gradually increase,
At about 1 M⊙, the gravitation force overcomes the electron degenerate pressure, and the white dwarf collapses, increasing temperature and density until it reaches carbon fusion temperature.
The carbon inside the white dwarfs are simultaneously ignited. It explodes to form a
White dwarf supernova. (Type I).
Nothing is left behind from a white dwarf supernova explosion (In contrast to a massive-star supernova, which would leave a neutron star or black hole behind). All the materials are dispersed into space.
White Dwarf Supernova is a very important standard candle for measuring cosmological distance…

15. White Dwarf and Massive Star Supernovae
Because the mass of white dwarfs when they explode as supernovae is always around 1.0 M⊙, its luminosity is very consistent, and can be used as a standard candle for the measurement of distance to distant galaxies (Chapter 15).
The amount of energy produced by white dwarf supernovae and massive star supernovae are about the same. But the properties of the light emitted from these two types of supernovae are intrinsically different, allowing us to distinguish them from a distance.
Massive star supernovae spectrum is rich with hydrogen lines (because they have a large outer layer of hydrogen).
White dwarf supernovae spectra do not contain hydrogen line (because white dwarfs are mostly carbon, with only a thin shell of hydrogen).
The light curve is different.

16. Supernovae from Distant Galaxies
These snapshots, taken by NASA's Hubble Space Telescope, reveal five supernovae, or exploding stars, and their host galaxies.
The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. The supernovae exploded between 3.5 and 10 billion years ago.

17. Distance and Redshift
In addition to distance, Hubble also measured the redshift of the galaxies…and when combined with distances derived from observation of Cepheid variables and the brightest stars in galaxies, Hubble found that, the more distant a galaxy, the greater its redshift is, and hence the faster it is moving away from us…
the universe is expanding!

18. H0 = 20~24 [km/sec] / million light-year
Hubble’s Law
From the redshift and distance measurements, we can express the recession speed V of a galaxy located at a distant d away from us by
V = d  H0
The value of the Hubble’s Constant is
H0 = 20~24 [km/sec] / million light-year
Once the value of H0 is determined, we can use measured recession speed to infer the distance of galaxies using the formula
d = V / H0