Astronomy Chapter 17: Galaxies

Galaxies

Categorized by their shapes.

Clusters of galaxies

Contain huge amounts of dark matter.

Superclusters

Formed by clusters of galaxies.

Spiral Galaxies

Classified by the tightness of their spiral arms and the sizes of their central bulges.

Sa galaxies

Have the largest central bulges and the most tightly wound spiral arms.

Sc galaxies

Have the smallest central bulges and the least tightly wound arms.

Andromeda (M31)

The only galaxy easily visible to the naked eye from Earth's northern hemisphere.

Andromeda's distance

Located only 2.5 million light-years (0.77 Mpc) from us.

Chandra X-ray Telescope

Discovered 26 stellar-remnant black holes in Andromeda.

NGC 7814

Classified as an Sa galaxy due to its large central bulge.

Sb galaxy

Characterized by a smaller central bulge.

Sc galaxy

Obscured central bulge due to interstellar dust.

Flocculent spiral galaxy

Has fuzzy, poorly defined spiral arms.

Grand-design spiral galaxy

Has well-defined spiral arms.

Rotation curve of disk stars

Indicates most stars have the same linear speed.

Spiral shape formation

Outer stars are left behind, creating a spiral shape.

Ripples in Water

Circular ripples expanding from a disturbance.

Spiral arms in galaxies

Created by ripples in the gas and dust of a disk galaxy.

Density wave in traffic flow

Moves rigidly around while stars and gas pass through it.

Gravitational force in density waves

Creates more gravitational force, compressing gas and initiating star formation.

O and B stars

Highlight the spiral density wave in a grand-design galaxy.

Barred Spiral Galaxy

A spiral galaxy with a central bar-shaped structure composed of stars.

SBa Galaxy

A barred spiral galaxy with the most tightly wound spiral arms and largest central bulges.

SBb Galaxy

A barred spiral galaxy with moderately tight spiral arms and medium-sized central bulges.

SBc Galaxy

A barred spiral galaxy with the least tightly wound spiral arms and the smallest central bulges.

Giant Elliptical Galaxy

A large elliptical galaxy, such as M84 and M86, found in the Virgo cluster.

Virgo Cluster

A rich collection of more than 2000 galaxies centered about 54 million light-years from Earth.

Dwarf Elliptical Galaxy

A small elliptical galaxy, such as Leo I, which is about 600,000 light-years from Earth.

E4 Galaxy

A classification of a dwarf elliptical galaxy, such as Leo I, which is 3000 light-years in diameter.

Elliptical Galaxy

A type of galaxy classified by how round or elongated it appears.

E0 Galaxy

An elliptical galaxy that is perfectly round.

E7 Galaxy

A very elongated elliptical galaxy.

Irregular Galaxy

A galaxy that does not have a distinct regular shape, such as the Large Magellanic Cloud.

Large Magellanic Cloud (LMC)

An Irr I irregular galaxy located 179,000 light-years away, spanning 62,000 light-years across.

H II Region

A region of ionized hydrogen, such as the Tarantula Nebula, with a diameter of 800 light-years.

Ultradiffuse Galaxy

A galaxy like Dragonfly 44, which is much dimmer than its neighboring galaxies.

Hubble's Tuning-Fork Diagram

A classification scheme for galaxies summarizing their types based on shape and structure.

Lenticular Galaxy

An S0 or SB0 galaxy that has a disk and a bulge, but no spiral arms.

Mass of Elliptical Galaxies

Ranges from 10^9 to 4 × 10^11 solar masses (Mʘ).

Luminosity of Irregular Galaxies

Ranges from 3 × 10^5 to 10^11 solar luminosities (Lʘ).

Diameter of Spiral Galaxies

Ranges from 3 × 10^3 to 3 × 10^4 light-years.

Population I Stars

Young stars typically found in the disk of galaxies.

Population II Stars

Older stars found in the central bulge and halo of galaxies.

Hercules Cluster

A group of galaxies located about 650 million light-years from Earth.

Voids

Large, relatively empty regions between superclusters.

Walls

Flatter distributions of galaxies compared to filaments.

Distribution of galaxies

This map shows the distribution of 62,559 galaxies in two wedges extending in opposite directions from Earth out to distances of 3.25 billion light-years.

Computer simulation of galaxies

This computer simulation shows how the galaxies, clusters of galaxies, and superclusters of galaxies are distributed on a grand scale.

Volume of the universe

The image represents how a volume of the universe 1.2 billion light-years across would appear.

Clusters and superclusters

The bright regions where the filaments of galaxies meet are where clusters and superclusters reside.

Local Group

The Milky Way Galaxy belongs to a poor, irregular cluster that consists of about 50 galaxies, called the Local Group.

Distribution of galaxies in the Local Group

This map shows the distribution of about three-quarters of the galaxies.

Largest galaxies in the Local Group

The Milky Way and Andromeda galaxies are the largest and most massive galaxies in the Local Group.

Satellite galaxies

Andromeda (M31) and the Milky Way are each surrounded by a dozen satellite galaxies.

Canis Major Dwarf Galaxy

The recently discovered Canis Major Dwarf Galaxy is the Milky Way's nearest known neighbor.

Antlia Galaxy

The galaxy Antlia was first detected in 1997. It lies about 3 million light-years away, outside the region depicted in Figure 17-20.

Stars in Antlia

This galaxy contains only about a million stars.

Coma Cluster

This rich, regular cluster, containing thousands of galaxies, is about 300 million light-years from Earth.

Regular clusters

Regular clusters are composed mostly of elliptical and lenticular galaxies and are sources of X-rays.

Coma's central region

This Chandra image shows Coma's central region, which is 1.5 million light-years across.

Gas cloud temperature

The gas cloud emitting most of these X-rays has a temperature of 100 million K.

Cartwheel Galaxy

A composite image of the Cartwheel Galaxy. This ring-shaped assemblage 500 million light-years from Earth is likely the result of one galaxy having passed through the middle of the larger one.

Star formation in Cartwheel Galaxy

This wave stimulated a burst of star formation, creating many bright blue and white stars.

Andromeda Galaxy

Infrared image of the Andromeda Galaxy. The ring of hot dust indicates star formation, probably caused by the passage of another galaxy through Andromeda.

Starburst Galaxy

NGC 1512, located 30 Mly away in the constellation Horologium, is 70,000 ly across.

Star formation in NGC 1512

A ring of vigorous star formation 2400 light-years wide highlights this ultraviolet, visible light, and infrared composite image of the core of this galaxy.

M81 Group

The Irr II starburst galaxy M82 is in a nearby cluster of about a dozen galaxies, including the spectacular spiral M81.

Streamers of hydrogen gas

Several of the galaxies in this cluster are connected by streamers of hydrogen gas.

Hydrogen gas

Large volumes of hydrogen gas, in red, being ejected from M82.

Very Large Array

A radio image created from data taken by the Very Large Array shows streamers of hydrogen gas connecting bright and dim galaxies.

NGC 4676

Known as 'the Mice', this system exhibits long 'antennae' of stars ejected by the collision of two galaxies, located 300 million light-years from Earth in the constellation Coma Berenices.

Star formation

The collision of galaxies stimulates a firestorm of new star formation, visible in bright blue regions.

Mass flow between galaxies

Mass can be seen flowing between colliding galaxies, which will eventually merge.

NGC 2207 and IC 2163

These two galaxies are orbiting and tidally distorting each other, with a close encounter occurring 40 million years ago.

Galactic diameter

During their close encounter, NGC 2207 and IC 2163 were about 1 galactic diameter apart.

NGC 6240

This object is the result of two spiral galaxies merging, located 330 million light-years from Earth in the constellation Ophiuchus.

Burst of star formation

The collision between two galaxies in NGC 6240 has triggered an immense burst of star formation.

Supermassive black holes

At the heart of NGC 6240 are two supermassive black holes, one from each original galaxy, expected to merge within a few hundred million years.

NGC 3393

Located half the distance from Earth as NGC 6240, this galaxy has maintained its spiral structure during a merger that occurred 1 billion years ago.

Black holes in NGC 3393

The two black holes in NGC 3393 are only 490 light-years apart and are expected to merge within another billion years.

Simulated Galactic Cannibalism

A computer simulation shows a small galaxy being devoured by a larger, disk-shaped galaxy.

Spiral arms generation

Spiral arms are generated in the disk galaxy by its interaction with the satellite galaxy.

Rotation curves

The graph shows how the orbital speed of material in the disks of four spiral galaxies varies with the distance from the center.

Flat rotation curves

Many galaxies have flat rotation curves that do not fall off, indicating the presence of extended halos of dark matter.

Gravitational lens

A schematic showing how a gravitational lens works, where light from a distant object changes direction due to gravitational attraction.

Examples of gravitational lensing

1. A blue ring galaxy lensed by a redder elliptical galaxy; 2. A pair of bluish images of the same object lensed symmetrically; 3. A lensed object appearing as a blue arc under the influence of a group of four galaxies.

Gravitational lensing image

An image showing the gravitational lensing of a distant galaxy and a quasar by a cluster of galaxies between them and us.

Gravitational Lensing

A phenomenon where the light from distant galaxies is bent due to the gravitational field of a massive object, such as a cluster of galaxies.

Dark Matter

A form of matter that does not emit light or energy, making it invisible and detectable only through its gravitational effects.

Hot Gas in Galaxy Clusters

The hot gas found in galaxy clusters, which can be imaged by the X-rays it emits.

Recessional Velocity

The speed at which a galaxy is moving away from an observer, calculated from the Doppler shifts of spectral lines.

Hubble Law

A formula that states the recessional velocity of a galaxy is proportional to its distance from us: Recessional velocity = H0 x distance.

Hubble Flow

The observation that galaxies are receding from each other at a rate proportional to their distance, indicating the expansion of the universe.

Megaparsec (Mpc)

A unit of distance used in astronomy, equivalent to one million parsecs, often used to measure distances between galaxies.

Doppler Shift

The change in frequency or wavelength of light from an object due to its motion relative to the observer.

Galaxy Clusters

Large groups of galaxies bound together by gravity, often containing hundreds to thousands of galaxies.

Spectra of Galaxies

The light spectrum emitted by galaxies, which can provide information about their composition, distance, and motion.

Redshift

The phenomenon where light from an object is shifted to longer wavelengths due to the object's motion away from the observer.

Intergalactic Gas

Gas that exists in the space between galaxies, often at very high temperatures.

Filament of Dark Matter

A structure formed by dark matter that connects galaxy clusters and can be revealed through gravitational lensing.