Galaxy and Milkyway Quiz

In 1924, Edwin Hubble classified galaxies based on what?

Their appearance.

Describe spiral galaxies.

Flattened disks with a central bulge, spiral arms containing gas, dust, and young stars.

Spiral Galaxy Type - Sa

Large bulge, tightly wound arms.

Spiral Galaxy Type - Sb

Medium bulge, moderately wound arms.

Spiral Galaxy Type - Sc

Small bulge, loosely wound arms.

Which type of spiral galaxy has the most interstellar matter?

C. Sc

If a spiral galaxy is tilted, how can we tell it’s a spiral?

By detecting its structure through observations of gas, dust, and star patterns.

Describe barred-spiral galaxies.

Similar to spiral galaxies but with a bar-shaped central structure extending from the bulge.

Barred-Spiral Galaxy Type - SBa

Large bulge, tightly wound arms.

Barred-Spiral Galaxy Type - SBb

Medium bulge, moderately wound arms.

Barred-Spiral Galaxy Type - SBc

Small bulge, loosely wound arms.

What type of barred-spiral galaxy is the Milky Way Galaxy believed to be?

E. SBb

Describe elliptical galaxies.

Smooth, featureless light profile; round to elongated in shape.

Ellipticals are classified based on how __ they are.

Elongated; E0 corresponds to circular, E7 to the most elongated.

Do elliptical galaxies have ongoing star formation? Why?

No; they lack gas and dust needed for new stars.

What are lenticular galaxies?

Galaxies with a central bulge and a disk but no spiral arms.

Lenticular galaxies are denoted as

S0 (no bar), SB0 (with bar).

Describe irregular galaxies.

Galaxies with no regular shape or structure.

The majority of galaxies in the universe are categorized as what?

Irregular.

Irregular Galaxy Type - Irr I

Some structure, possible remnants of collisions.

Irregular Galaxy Type - Irr II

Chaotic, no structure.

What are the Magellanic Clouds?

Two irregular dwarf galaxies near the Milky Way.

What did Hubble originally think about galaxy evolution?

Galaxies started as ellipticals and evolved into spirals.

How do we know galaxies don't evolve as Hubble thought?

Observations show young spirals and old ellipticals exist.

The most elongated ellipticals are:

E7.

An intermediately-wound barred-spiral galaxy is:

SBb.

Why can’t we study motions of gas, dust, and stars using optical wavelengths?

Because dust blocks optical light.

What kind of wavelengths do we study instead?

Radio wavelengths.

What have measurements of gas, dust, and stars revealed to astronomers?

The structure and rotation of the Milky Way.

What kind of galaxy is the Milky Way Galaxy?

A barred spiral galaxy.

How far is Earth from the center of the Galaxy?

About 26,000 light-years.

What bright objects make spiral arms visible from afar?

Young, bright, massive stars and nebulae.

How do the spiral arms stay in place if they move differently than the disk?

They are density waves, not material structures.

Where does most star formation happen in the Galaxy?

In the spiral arms.

The spiral arms revolve more slowly than the disk as __, with stars moving in and out like cars in a __.

Density waves; traffic jam.

What is self-propagating star formation?

Star formation triggered by shock waves from nearby supernovae, leading to new star formation and spiral patterns.

What two measurements do we need to use Kepler’s third law for Galactic mass?

Orbital period and orbital radius.

What does the rotation curve of the Milky Way suggest?

Mass is spread out and doesn't drop off as expected — indicating dark matter.

What is believed to exist in the outer regions of our Galaxy?

Dark matter.

Why is dark matter called "dark"? How can we detect it?

It doesn’t emit light; detected through gravitational effects.

Dark Matter Candidate - Primordial black holes

Tiny black holes from the early universe.

Dark Matter Candidate - MACHOs

Massive objects like brown dwarfs or black holes.

Dark Matter Candidate - WIMPs

Hypothetical particles interacting only weakly with matter.

Dark Matter Candidate - Axions

Extremely light particles theorized in physics.

What is the leading candidate for dark matter?

WIMPs (Weakly Interacting Massive Particles).

What constellation should you look at to see the Galactic Center?

Sagittarius.

What is in the center of our Galaxy?

A supermassive black hole.

What is the black hole called?

Sagittarius A* (Sagittarius A-star).

What is the mass of Sagittarius A*?

About 4 million solar masses.

How did astronomers determine the mass of Sagittarius A*?

By measuring the orbits of nearby stars.

What is a galaxy?

A collection of stars, gas, dust, and dark matter bound by gravity.

What is the name of our Galaxy?

The Milky Way.

Where does Earth reside within the Galaxy?

In the Orion Arm (local spur) of the Galactic disk.

What kind of galaxy is the Milky Way? Which nearby galaxy is similar?

Barred spiral; similar to the Andromeda Galaxy.

What did William Herschel do to measure the Galaxy?

Mapped star counts in different directions.

Why was Herschel’s method flawed?

He didn't account for interstellar dust blocking starlight.

To better understand the Galaxy's size, what was needed?

A better distance indicator (like variable stars).

What are cataclysmic variables?

Stars that undergo sudden, dramatic changes in brightness.

What are intrinsic variables?

Stars that vary in brightness due to internal processes.

Intrinsic Variable Star - RR Lyrae

Short period (~0.5 day), lower luminosity.

Intrinsic Variable Star - Cepheid

Longer period (1–100 days), high luminosity.

Are intrinsic variable stars in hydrostatic equilibrium?

No.

Where are intrinsic variable stars found on the H-R diagram?

Instability strip.

Most variable stars were once __ main sequence stars.

Massive.

How long does the intrinsic variable phase last?

A brief phase during stellar evolution.

Why are Cepheid variable stars useful to astronomers?

Their period relates to their luminosity, allowing distance measurements.

Once we learn a Cepheid’s luminosity from its period, what can we calculate?

Its distance.

Variable stars allow distance measurement out to __.

Other galaxies.

What did Harlow Shapley do to measure the Galaxy's size?

Used globular clusters and RR Lyrae variables to find the center.

Why was Shapley’s method better than Herschel’s?

It wasn’t blocked by dust — he measured using globular clusters.

Spiral Galaxy Components - Galactic halo

Spherical shape, old stars, little gas/dust, no star formation, random orbits, reddish color.

Spiral Galaxy Components - Galactic disk

Thin/flat, young and old stars, contains gas/dust, ongoing star formation, circular orbits, bluish color.

Spiral Galaxy Components - Galactic bulge

Spherical/elliptical shape, old stars, little gas/dust, no star formation, random and some circular orbits, reddish color.

The formation of the Galaxy is believed to be similar to the formation of the __.

Solar System.

Why can we use Type Ia supernovae to measure distances up to 1 Gpc?

They have a consistent peak brightness — excellent standard candles.

What are standard candles?

Objects of known luminosity used to measure distances.

What is the Tully-Fisher relation?

A relation between a spiral galaxy's luminosity and its rotation speed.

What is the Local Group?

A group of about 50 galaxies including the Milky Way, Andromeda, and Triangulum.

What is a galaxy cluster?

A large collection of galaxies bound together by gravity.

Outside the Local Group, the next galaxy cluster we reach is?

The Virgo Cluster.

What is Abell 1689?

A massive galaxy cluster used for gravitational lensing studies.

How do galaxy “groups” compare to galaxy “clusters”?

Groups are smaller; clusters are larger.

What does Hubble’s Law state?

The farther away a galaxy is, the faster it is moving away from us.

How do galaxies in groups or clusters move?

They orbit the cluster's center of mass and can move toward or away from each other.

What is redshift?

The lengthening of light waves as an object moves away, making light appear redder.

What does Hubble’s Law imply about the universe?

The universe is expanding.

Are the sizes of the Solar System, Milky Way, galaxies, and clusters increasing?

No; only the distances between clusters are increasing.

What is the meaning of Hubble’s constant?

The rate of expansion of the universe.

What is the currently accepted value of Hubble’s constant?

About 70 km/s/Mpc.

Hubble’s law allows measurement of distances to the __ galaxies.

Most distant.

According to Hubble’s law, the greater a galaxy’s redshift, the:

Farther away it is.

What are active galaxies compared to normal galaxies?

Emit more energy, often in non-visible wavelengths.

Active galaxies emit the bulk of their light in __ parts of the electromagnetic spectrum, due to what?

Non-visible; due to material falling into a supermassive black hole.

Nuclei of active galaxies are called __.

Active Galactic Nuclei (AGN).

How do Seyfert galaxies differ from normal galaxies?

They look normal but have very bright cores and emit a lot of infrared.

The majority of Seyfert galaxies emit most of their light as __, why?

Infrared light, because of warm dust heated by the nucleus.

Radio galaxies emit radio waves from where?

From huge jets and lobes outside the galaxy.

__ radio galaxies have radio emission from their centers, but it might be projection effects.

Compact.

What are quasars?

Quasi-stellar objects with very high redshifts, extremely luminous and distant.

Quasars are the __ distant objects we know.

Most.