The Galaxy. (The Milky Way Galaxy)

The Galaxy (The Milky Way Galaxy)

Which is a picture of the Milky Way? A A is what we see from Earth inside the Milky Way while B is what the Milky Way might look like if we were far away looking back at our own galaxy from some other galaxy. B

The Milky Way is composed of all the stars in our galaxy, nearly 400 billion. All the stars you can see in the sky are in our Galaxy. Enormous clouds of dust obscure our view of most of the stars in our Galaxy

Interstellar dust hides the true extent of the Milky Way This dust makes it difficult to determine distances to stars because dust impedes our accurate measurement of brightness. This dust also makes it nearly impossible to see what is at the center of our galaxy.

The Interstellar Medium (ISM) It is the stuff between the stars. It is mostly a vacuum (1 atom cm-3). It is composed of 90% gas and 10% dust. gas: individual atoms and molecules dust: large grains made of heavier elements The ISM effectively absorbs or scatters visible light. it masks most of the Milky Way Galaxy from us Radio & infrared light can pass through the ISM. we can study and map the Milky Way Galaxy by making observations at these wavelengths

Distance measurements to globular clusters define the location of the galactic center. Globular clusters form a sphere around the center of the Milky Way. In 1917, Harlow Shapley determined distances to globular clusters by finding variable stars with known absolute magnitude.

The center of the distribution of globular clusters shows the location of the Milky Way s center. globular clusters galactic nucleus nuclear bulge spiral arms disk note position of the Sun, just over half way out. Sun Nuclear bulge Galactic nucleus Disk Globular Clusters

Regions of the Milky Way Galaxy diameter of disk = 100,000 l.y. (30,000 pc) thickness of disk = 1,000 l.y. (300 pc) number of stars = 400 billion Sun is in disk, 28,000 l.y. out from center

Disk Regions of the Milky Way Galaxy younger generation of stars contains gas and dust location of the open clusters Bulge mixture of both young and old stars Halo older generation of stars contains no gas or dust location of the globular clusters

The matter in our Galaxy emits different kinds of radiation.

The ground state in hydrogen is actually two energy levels and if it is left alone long enough λ= 21 cm And there s lots of Hydrogen out there!!!

Radio observations help map the Looking for 21-cm wavelengths of light emitted by interstellar hydrogen as we look along the disk of the Milky Way (from inside), we see 21-cm photons Doppler shifted varying amounts this allows the interstellar hydrogen to be mapped galactic disk

A Map of the Milky Way Based on 21-cm wavelength light mapping

Zone of Avoidance

Spiral Galaxy M83 observed in both visible light and radio wavelengths.

Don t galaxies look like they spin?

d We investigate the Rotation Curve of our Galaxy by plotting the velocity as a function of radial distance d from the center.

Orbital Velocities in the Disk rotation curve a plot of rotational (orbital) speed.vs. distance from the center speed radius

Do galaxies rotate like a Merry-Go-Round? (solid body rotation)

Differential Rotation of the Galaxy The Sun orbits at 230 km/s or about 500,000 mph

Do they rotate like planets in our solar system? (Keplerian falloff)

The Galaxy s Rotation Curve

Most of the matter in the Galaxy has not yet been identified According to Kepler s Third Law, the farther a star is from the center, the slower it should orbit Observations show that speed actually increases with distance from the center This could be due to gravity from extra mass we cannot see - called DARK MATTER.

Orbital Velocities in the Disk rotation curve a plot of rotational (orbital) speed.vs. distance from the center speed radius

Halo Zone of Avoidance

The Very Large Array (VLA) in New Mexico

Center of the Galaxy Radio Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*. X-ray

Center of the Galaxy in Sagittarius Infrared Visual

Center of the Galaxy Radio Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*. X-ray

X-ray Flare from Sgr A* Chandra image of Sgr A* The rapid flare rise/drop time (< 10 min) implied that the emission region is only 20 times the size of the event horizon of the 2.6 million M black hole. Observations are consistent with the existence of a supermassive black hole at the center of our Galaxy. Energy from flare probably came from a comet-sized lump of matter torn apart before falling beneath the event horizon!

The Star Gas Star Cycle

Halo vs. Disk Stars Stars in the disk are relatively young. fraction of heavy elements same as or greater than the Sun plenty of high- and low-mass stars, blue and red Stars in the halo are old. fraction of heavy elements much less than the Sun mostly low-mass, red stars Stars in the halo must have formed early in the Milky Way Galaxy s history. they formed at a time when few heavy elements existed there is no ISM in the halo star formation stopped long ago in the halo when all the gas flattened into the disk

Stellar Orbits in the Galaxy Stars in the disk all orbit the Galactic center: in the same direction in the same plane (like planets do) they bobble up and down this is due to gravitational pull from the disk this gives the disk its thickness Stars in the bulge and halo all orbit the Galactic center: in different directions at various inclinations to the disk they have higher velocities they are not slowed by disk as they plunge through it nearby example: Barnard s Star

Mass of the Galaxy We can use Kepler s Third Law to estimate the mass Sun s distance from center: 28,000 l.y. = 1.75 x 109 AU Sun s orbital period: 230 million years (2.3 x 108 yr) P2 = 4 2/GM a3 mass within Sun s orbit is 1011 M Total mass of MW Galaxy : 1012 M Total number of stars in MW Galaxy 2 x 1011

Galaxies

Galaxies seem to take one of four different appearances I. SPIRALS

SPIRALS

The tightness of a spiral galaxy s arms is correlated to the size of its nuclear bulge Type Sa Type Sb Type Sc

Variety of Spiral Arms Flocculent spirals (fleecy) Grand-design spirals (highly organized)

We easily see these spiral arms because they contain numerous bright O and B stars which illuminate However, stars in total seem to be evenly distributed throughout the disk. dust in the arms.

Spiral Structure Our Galactic disk does not appear solid. it has spiral arms, much like we see in other galaxies like M51 These arms are not fixed strings of stars which revolve like the fins of a fan. They are caused by compression waves which propagate around the disk. such waves increase the density of matter at their crests we call them density waves M 51 they revolve at a different speed than individual star orbit the Galactic center Note how the spiral arms appear bluer compared to the bulge or the gaps between the arms.

Compression Wave

Spiral Arms The compression caused by density waves triggers star formation. molecular clouds are concentrated in arms plenty of source matter for stars short-lived O & B stars delineate the arms and make them blue & bright long-lived low-mass stars pass through several spiral arms in their orbits around the disk

Galaxies seem to take one of four different appearances I. SPIRALS

BARRED SPIRALS

Bars of stars run through the nuclear bulges of barred spiral galaxies Type SBa Type SBb Type SBc

Galaxies seem to take one of four different appearances I. SPIRALS

ELLIPTICALS

Elliptical galaxies display a variety of sizes and masses Giant elliptical galaxies can be 20 times larger than the Milky Way Dwarf elliptical galaxies are extremely common and can contain as few as a million stars

Galaxies seem to take one of four different appearances I. SPIRALS

This classification scheme is known as the Hubble Tuning Fork Scheme