Astronomy Chapter 16: The Milky Way Galaxy

Milky Way Galaxy

Billions of stars along with stellar remnants, gas, dust, and dark matter are all bound together by their mutual gravitational attraction.

Dark Matter

A significant fraction of the Milky Way's total mass that has yet to be identified.

Structure of the Milky Way Galaxy

The Galaxy has two major arms and several shorter arm segments, all spiraling out from the ends of a bar of stars and gas that passes through the Galaxy's center.

Supermassive Black Hole

Located at the center of the Milky Way Galaxy.

Schematic Diagrams of the Milky Way

Simplified edge-on drawing shows the Milky Way's disk, containing most of its stars, gas, and dust, and its halo, containing many old stars.

Lord Rosse's Telescope

Built in 1845, housed a 1.8-m-diameter telescope, the largest of its day, providing improved resolution over other telescopes.

M51

Also called NGC 5194, known as the Whirlpool Galaxy, located about 20 million light-years from Earth.

Cepheid Variable Stars

One of the most reliable ways to determine the distance to nearby or moderately remote galaxies.

Distance to Galaxy M100

Determined to be 50 million light-years (15.2 Mpc) from Earth using Cepheid variable stars.

Period-Luminosity Relation

Graph showing the relationship between the periods and average luminosities of classical (Type I) Cepheid variables and RR Lyrae stars.

Brightness of Stars in the Milky Way

This image shows the locations and brightnesses of nearly 1.7 billion stars in the Milky Way Galaxy.

Galactic Center

The center of the Galaxy is in the constellation Sagittarius.

Interstellar Clouds

Dark lines and blotches caused by hundreds of interstellar clouds of gas and dust that obscure the light from background stars.

Globular Clusters

Prominent clusters visible beyond the disk stars in the Milky Way.

Electron Spin

Due to their spins, electrons and protons both act as tiny magnets.

Higher-energy configuration

When an electron and the proton it orbits are spinning in the same direction.

Lower-energy configuration

When the electron flips from the higher-energy to the lower-energy configuration, the atom loses a tiny amount of energy that is radiated as a radio photon with a wavelength of 21 cm.

Doppler shifts

The slight variations in radio waves from gas clouds that allow astronomers to sort out the gas clouds and map the Galaxy.

21-cm radiation

Radiation used in radio telescope surveys to map the distribution of hydrogen gas in the Galaxy.

Local arm

A minor arm of the Milky Way where our solar system lies, also known as the Orion arm or the Orion-Cygnus arm.

Sagittarius arm

One of the two major arms of the Milky Way Galaxy.

Perseus arm

The other major arm of the Milky Way Galaxy.

M83

A galaxy located in the southern constellation of Centaurus, about 12 million light-years from Earth.

Population I stars

Young, metal-rich stars found in the disk of the Milky Way Galaxy.

Population II stars

Old, metal-poor stars that compose the halo of the Milky Way Galaxy.

Diameter of the Milky Way

About 100,000 light-years.

Distance from the solar system to the galactic center

About 26,000 light-years.

Infrared image from COBE satellite

An image taken in 1997 showing the disk and central bulge of the Galaxy, with nearby stars appearing white and interstellar dust appearing orange.

X-shaped distribution of stars

A hint of the arrangement of stars in the central bulge of the Milky Way as seen in infrared images.

Magnetic field of the Milky Way

The magnetic field in and around the Galaxy that causes interstellar dust to be oriented in certain ways.

Polarizing dust

Dust that causes light scattering off it to be polarized, with color variations indicating the amount of dust.

Color representation of dust

Darker orange and brown show the most dust, while blue shows the least.

Magnetic field direction

The curves indicate the direction of the magnetic field.

Infrared emission

Black represents the dimmest regions of infrared emission, with blue the next strongest, followed by yellow and red; white represents the strongest emission.

Dust layer

The prominent band diagonally across this photograph is a layer of dust in the plane of the Galaxy.

Sagittarius A*

This infrared image shows about 300 of the brightest stars less than 1 ly from Sagittarius A*, which is at the center of the picture.

Star density

The distribution of stars and their observed motions around the galactic center imply a very high density (about a million solar masses per cubic light-year) of less luminous stars.

X-ray flare

An X-ray flare from a Sagittarius A* image in 2012, by NuSTAR (Nuclear Spectroscopic Telescope Array).

Radio image

A radio image taken at the VLA of the galactic nucleus and environs covers an area of the sky 8 times wider than the Moon.

SNR

SNR means supernova remnant.

Star orbits

The colored dots superimposed on this infrared image show the positions over time of 10 stars in the vicinity of the unseen massive object at the position of the radio source Sagittarius A*.

Orbit measurement period

The orbits were measured over a 21-year period.

Fastest known orbit

The star with the fastest known orbit goes around once every 11.5 years.

Black hole mass

This plot indicates that the stars are held in orbit by a 4.3 x 10^6 Mʘ black hole.

Gamma-ray bubbles

Two gamma-ray-emitting bubbles of hot gas extend 23,000 ly above the plane of the Galaxy.

Gas and dust in bubbles

They each contain about 2 x 10^6 Mʘ of gas and dust.

Jet of gas

A jet of gas leaving the Milky Way's nucleus creates shock fronts when interacting with other gas in the region.

Disk galaxy M58

This disk galaxy, M58, looks very similar to what the Milky Way Galaxy would look like from far away.

Star orbits in components

The colored arrows show typical orbits of stars in the central bulge (blue), disk (red), and halo (yellow).

Canis Major Dwarf Galaxy

The Canis Major Dwarf Galaxy is a dwarf elliptical galaxy that lies some 25,000 ly from the Milky Way.

Gravitational tidal force

The distribution of stars being stripped from the Canis Major Dwarf Galaxy is due to our Galaxy's gravitational tidal force.

Star count in Canis Major

Containing only about 1 billion stars, the Canis Major Dwarf will be completely pulled apart within the next 100 million years or so by the Milky Way.

Differential rotation

If all stars in the Galaxy had the same angular speed, they would orbit in lockstep.

Galaxy's Rotation Curve

The blue curve shows the orbital speeds of stars and gas in the Galaxy, and the dashed red curve shows Keplerian orbits that would be caused by the gravitational force from all the known objects in the Galaxy.

Gravitational Microlensing

Gravitational fields cause light to change direction, making distant objects appear brighter than they are normally.

Light Curve

The light curve of the gravitational microlensing of light from a star in the Galaxy's central bulge by an intervening object.

Milky Way Debate

The inconclusive Shapley-Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe.

Diameter of the Galaxy

Our Galaxy has a disk about 100,000 ly in diameter and about 2000 ly thick.

Interstellar Dust

Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths.

21-cm Radio Waves

Hydrogen clouds can be detected beyond interstellar dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds.

Galactic Nucleus

The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths.

Central Bulge

The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called the central bulge.

Spiral Arms

A disk with two distinct arms and several less complete arms of stars, gas, and dust spirals out from the ends of the bar in the galactic central bulge.

Young OB Associations

Young OB associations, H II regions, and molecular clouds in the galactic disk highlight the spiral arms where stars are forming.

Sun's Location

The Sun is located about 26,000 ly from the galactic nucleus, between the spiral arms.

Sun's Orbit Speed

The Sun moves in its orbit at a speed of about 860,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy.

Inner Halo

The inner halo includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter.

Outer Halo

The outer halo is composed of dark matter and very old stars, which have retrograde orbits.

Distance modulus

The difference between the apparent and absolute magnitudes of an object.

Galactic cannibalism

A collision between two galaxies of unequal mass and size in which the smaller galaxy is absorbed by the larger galaxy.

Galaxy

A large assemblage of stars, gas, and dust bound together by their mutual gravitational attraction.

Halo (of a galaxy)

A spherical distribution of globular clusters, isolated stars, and dark matter that surrounds a galaxy.

Nebula (plural, nebulae)

A cloud of interstellar gas and dust

Shapely-Curtis debate

An inconclusive debate between Harlow Shapley and Heber Curtis in 1920 about whether certain nebulae (now known to be galaxies) are outside the Milky Way.

Synchrotron radiation

The radiation emitted by charged particles moving through a magnetic field; nonthermal radiation.