Stars, Galaxies, and Cosmology – EXAM 2

Why are spiral galaxies often called “disk galaxies”?

Most of their mass is confined to a disk.

What is the multi-disk component of spiral galaxies?

The disk of spiral galaxies is composed of a thin disk and thick disk.

• The thin disk is a thin component at the mid-plane of the galaxy where new gas is and new stars are formed.

• The thick disk is a thicker component populated by mostly older stars.

What are grand design spiral galaxies?

They are spiral galaxies with well defined, symmetric spiral features.

What are flocculent spiral galaxies?

They are spiral galaxies with patchy regions in their spiral structure.

Where are the spiral arms contained in spiral galaxies?

They are contained within the disk of a given spiral galaxy.

What are bars and bulges?

They are central features to galaxies and contain an older population of stars. In the center of these regions, there can be an active supermassive black hole!

What are bulges?

They are spheroidal (with an emanating X pattern!), that exhibit some characteristic light fall off, and exist from formation until forever.

What are bars?

They are formed by both the internal and external dynamics of a galaxy, and exist on timescales to approximately a gigayear.

What is the galactic halo?

The diffuse, low-density stars around the disk of a galaxy, roughly distributed spherically. These stars are usually quite old and in the form of globular clusters, and make up 1% of the stars in a given galaxy.

What is the baryon cycle?

The paradigm where the diffuse, low-density gas in a galactic halo could be kicked out or falling into it, or both could be occurring simultaneously.

What are late type galaxies?

They are spiral (or irregular) galaxies classified with higher letters (A-D) on Hubble’s Classification Scheme.

Why are spiral galaxies blue and have long dust lanes?

Spiral galaxies have large amounts of gas, dust, and active star formation.

Why can we see neutral hydrogen (H1)?

We can see H1 because of the spin-flip transition that hydrogen atoms undergo due to the intrinsic magnetic dipole moments.

How can we measure how many O-stars are in a galaxy?

By measuring the amount of UV light (photons) coming towards us.

How can we measure how many OB-stars are in a galaxy?

By measuring the amount of dust emission coming towards us and extrapolating back to the initial star formation. These stars emit intense, hot radiation, which is absorbed and re-emitted by intervening dust.

How can we measure how many large mass OB-stars are in a galaxy?

By measuring radio emission and extrapolating back to the initial star formation.

• These stars eventually die and explode into supernova, releasing massive amount of energetic electrons that interact with the magnetic fields of the galaxy, producing radio raves.

What is the relationship between star-formation and gas content?

The more gas you have, the more stars you are making.

Why are elliptical galaxies so boring?

They are featureless. They have no areas of active star formation, no gas or dust features, and no structural defects or patterns.

What is the movement of stars in spiral galaxies v.s. elliptical galaxies?

Stars are rotating in spiral galaxies and moving in random directions in elliptical galaxies.

What are lenticular galaxies?

They are strange galaxies that exhibit properties of both spiral and elliptical galaxies.

• They are rotating, but not quite as fast as spiral galaxies.

• They are forming stars, but not too many.

• They have some discs structures, but they also have an elliptical style brightness profile.

• They have some gas, but not too much gas.

What are active galactic nuclei (AGN)?

They are incredibly luminous, compact regions at the center or nuclei of some galaxies, powered by supermassive black holes accreting surrounding matter.

• The power is provided via SMBH accretion.

• The types of AGN are determined by our viewing angle of the galaxy.

What are supermassive black holes (SMBH)?

Black holes (small, but super powerful) in the center of a galaxy. They are surrounded by an accretion disk, a dusty torus (donut), and gas clouds.

What are magellanic galaxies?

They are galaxies characterized by their bar-like structure, single spiral arm, and masses between dwarf galaxy and major galaxy. They are named after the Large Magellanic Cloud, and found near a larger spiral galaxy.

What is harassment?

When a galaxy is gravitationally distorted by another galaxy, affecting their properties.

What are ring galaxies?

They are galaxies thought to be formed by a smaller galaxy falling through the center of a larger galaxy.

What is a hydrodynamic interaction?

An interaction that occurs when a galaxy passes through some diffuse medium. As a result, gas in the galaxy can be removed or displaced.

What are jellyfish galaxies?

They are large spiral galaxies found on high-velocity radial orbits in galaxy clusters. They move through the center of galaxy clusters, and the hot gas in galaxy clusters removes gas from these galaxies in long tails that can then go on to form stars.

How does dark matter compare to normal baryonic matter?

It out numbers normal baryonic matter by approximately a factor of 5. It is much less dense than baryonic matter but it is pervasive in galaxy halos and in clusters of galaxies.

Where is dark matter in a galaxy?

It lives in a halo much larger, 5-10x, than the stellar halo and contains most of the mass of the galaxy.

What is a standard candle?

An object with a known intrinsic brightness that correlates with other observed properties.

What are cepheid variables?

Massive, luminous, and unstable stars that pulsate radially, changing in brightness and size over regular periods of days to months. Their pulsation period directly correlates to their intrinsic luminosity (brighter stars have longer periods).

What is redshift?

The stretching of light wavelengths toward longer, redder wavelengths as an object moves away from the observer, or as space expands.

What is “dark” energy (also known as “vacuum energy”)?

The energy associated with empty space and is responsible for the observed accelerating expansion of the universe.

What is the cosmic web of galaxies?

The coherent, large-scale structure of the universe’s galaxies.

What are galaxy clusters?

Galaxies are often found in clusters of galaxies, which are virtualized systems.

• They are dispersion supported and can lead to significant galaxy harassment.

• They contain hundreds to thousands of galaxies and a large amount of hot gas in their center.

• They contain a lot of dark matter.

• They are great laboratories for galaxy evolution!

What is ram pressure?

The force exerted on a body moving through a fluid medium (liquid or gas) due to its motion, representing the resistance of said medium.

What is the big bang nucleosynthesis (BBN)?

The process during recombination where the universe creates hydrogen (75% by mass), helium (25% by mass), and trace amounts of lithium (1 atom in a billion) + even less of any heavier elements.

What is the theory behind the first ever stars?

The hypothesis is that the first stars that emerged from the Dark Ages were massive stars, named Population III stars.

• These stars are theorized to have been extremely massive, about 100-1000x the mass of the Sun.

• They are way brighter and shorter-lived than the Sun!

• They have no metals since metals weren’t formed yet!

What is reionization?

The process by which electrically neutral hydrogen atoms in the early universe were ionized (reverted back into free protons and electrons) due to the increasing radiation from Population III stars.

What was the first light of the universe?

A cosmic microwave background!

What is the big bang theory?

The model explaining that the universe began approximately 13.8 billion years ago from an extremely hot, dense point (singularity) and has been expanding, cooling, and evolving ever since.

• The analogy is still simply the deflating of the balloon → extrapolating the expansion all the way back to a single point (initial singularity)

• Up until 10^{-43} seconds, the universe was 10^{32} K, all sources were unified, and who knows what the universe was like!

What does rapid inflation refer to?

The period of extremely rapid acceleration in growth of the universe → the universe grew rapidly in size (to approximately a meter) from 10^{-35} s to 10^{-33} s.

What is the theory of a “bouncy ball universe”?

If the expansion of the universe has the property that it begins to slow, it may be that it begins to force the universe to recoil / fold back in on itself.

• This theory helped solve the so-called horizon problem, but seems unlikely to occur given our current observations of the universe.

• This also helps us conceptualize a possibility of what a before the Big Bang could look like.

What is the theory of the “big rip”?

If dark energy has the property that its energy density increases as the universe expands, the entirety of the space-time fabric will rip.

• In light of current observations, this theory seems unlikely, but still, it cannot be ruled out.

What is the theory of the “heat death of the universe”?

If dark energy has the property that its energy density is constant or decreases as the universe expands, the entirety of the universe will simply fade.

• In light of current observations, this theory seems the most likely of all theories regarding the fate of the universe.

  • It is not a point for concern, since it will take a googol-ish ( 10^{100} ) years.