The Milky Way: Our Home Galaxy

Stellar populations & spiral structure:The bulge, disk, and halo

The Milky WayFrom our vantage point on Earth, it is difficult to get a sense of the size and shape of the Galaxy.

To ancient astronomers, the Milky Way was a luminous band of fluid(milk) crossing the sky.

Only when Galileo looked with his telescope, did he realize the glow was due to countless stars.

A Look at the Milky WayOur view from inside the Milky Way is remarkably similar to seeing another galaxy edge-on.

Image of spiral galaxy NGC 281 (WIYN telescope, Kitt Peak)

Axel Mellinger’s all sky mosaic of the Milky Way

A Look at the Milky WayWe see the most stars when we look through the Milky Way; we see few stars if we look away.

outward

outward

to centeraway from

center

away fromcenter

We get a better idea of the shape of the Milky Way by looking at other spiral galaxies that appear more face-on.

M51, the Whirlpool M83, Southern Pinwheel

A Look at the Milky Way

The Chemical History of the GalaxyStars enrich the Galaxy with heavy elements:

Globular ClustersThe most ancient populations of stars aren’t inside the Milky Way, but orbit around its center.

These globular clusters contain 100,000 – 1 million stars in a compact volume 100 light-years across.

Omega Centauri

16,000 light-years away

12-13 billion years old

Variable stars in cluster M3

Images taken over a single night at Whipple Observatory in southern Arizona. Credit: K. Stanek & A. Szentgyorgyi

In the 1910’s, Harlow Shapley used these variable stars to measure the distances to the globular clusters. He reasoned that they were clustered around the center of the Galaxy. He was right.

The Structure of the Milky Way

Globular clusters are the most prominent part of the halo of the Milky Way.

But they only make up 1% of the halo’s mass.

Most of the mass is made up of individual starsorbiting the Galaxy’s center.

There is very little gas or dust in the halo, so there is little or no ongoing star formation there. Objects in the galactic halo are very old.

The Structure of the Milky Way

The globular clusters and halo stars orbit around the center of the Galaxy, but randomly in direction.

The halo is roughly spherical and surrounds the Milky Way’s center.

The Structure of the Milky Way

The stars of the halo extend much farther outward from the center than the disk of the Galaxy does.

Open clusters are mostly confined to the main body or disk of the Milky Way. Here are the positions of the 100 brightest open clusters:

The Structure of the Milky Way

Open ClustersThe Pleiades is one example of an open cluster, a group of 1000’s of stars spread over a region about 10 light-years in diameter. This cluster, also known as M45, is in the constellation Taurus.

Open ClustersThese stars represent the population born in giant molecular clouds throughout the Milky Way.

The Eagle Nebula(aka M16) contains the young open cluster NGC 6611.

The cluster is still in the process of forming. The O,B stars light up the nebula.

Open ClustersOver time, the stellar winds and starlight blow away the remaining gas and dust leaving behind just the stars. The stars’ collective gravity holds the group together as a cluster.

The disk of the Galaxy is rotating. That is, all the matter in the disk is orbiting the center in the same direction.

The Structure of the Milky Way

The disk contains most of the visible mass of the Milky Way and nearly all of the gas and dust.

The Sun, a disk star, takes 220 million years to complete one orbit of the galactic disk.

The disk is the main part of the Milky Way that is actively forming new stars since it is the part that contains most of the dust and gas.

The stars of the disk are more enriched in heavy elements than the halo stars because they formed from gas that includes matter that was ejected by countless generations of dying stars.

The Structure of the Milky Way

The third component of the Milky Way is the bulge.

The bulge contains a mixture of young and old stars and some gas. It lies at the center of the Milky Way.

The stars in the bulge orbit randomly like the stars in the halo. But orbits inside the bulge are small and confined to the center of the Milky Way.

The Structure of the Milky Way

Here is the view of the Galaxy as seen in infrared(wavelengths about 3-4 times longer than visible):

The Structure of the Milky Way

central bulge

Halo Disk Bulge

shape spherical flattened football

orbits random circular random

age old young mixture

gas/dust very little a lot some

The Structure of the Milky Way

Estimated number of stars: 200 – 400 billion

Visible mass: 600 billion solar masses

Age: 13 billion years (oldest stars in halo)

8-10 billion years (oldest stars in disk)

Diameter: 100,000 light-years (stars)

200,000 light-years (gas)

Thickness of disk: 2000 light-years (stars)

10,000 light-years (gas)

The Structure of the Milky Way

Spiral Arms and the Sun’s PlaceWhile new data has refined its exact location in the Milky Way, it is clear that the Solar System lies in a relatively quiet portion of the galactic disk.

The Solar neighborhood lies between spiral arms.

The nearest populations of newborn stars are a few hundred light-years away and they are small groups.

Spiral ArmsThe spiral arms of a galaxy like the Milky Way are an overlying pattern on the disk of stars and gas.

The matter in the disk moves through the spiral arms. There is no matter permanently part of the spiral arms.

You can think of spiral arms as a traffic snarl in the orbits or gas and dust in the disk.

Imagine a multilane highway with the flow of traffic disturbed, for example, by a slow moving vehicle.

Heavy Traffic in the Spiral ArmsThe cars within the disturbance don’t stay there.

In the same way, stars and gas clouds are not a permanent part of a spiral arm.

Dense Environments in Spiral Arms

Stars and gas clouds gather in the spiral arm, and then leave again. The spiral pattern moves at a different speed than the stars and gas clouds.

What differences would we expect to see between the spiral arms and the inter-arm spaces?

The arms show young star clusters containing hot, blue stars and star-forming nebulas.

New Stars in Spiral ArmsBut hot blue stars are massive, so they live short lives. They don’t move far from the spiral arm before they die.

Here’s the center of M51 showing lots of bright blue stars and red star-forming nebulas along the spiral arms.

Also notice a lot of dust running along each arm. (Hubble Space Telescope)

Origin of Spiral ArmsWhat provides the “traffic snarl” that gets the spiral pattern going?

It appears to be the gravitational tug of smaller galaxies in orbit around a larger spiral galaxy.

The Milky Way has many companion galaxies.

The two largest are known as the Large and Small Magellanic Clouds (or LMC and SMC, for short).

They are named for explorer Ferdinand Magellan, but they are easily visible to the unaided eye so they were known to anyone south of the equator.

The Magellanic CloudsHere’s where they are located on the sky:

Large Magellanic Cloud

Small Magellanic Cloud

The Magellanic CloudsHere’s a closer view of the LMC. It is an example of an irregular galaxy. It has a mass of a few billion solar masses:

Giant star-forming nebula called the Tarantula Nebula or 30 Doradus

Supernova 1987A occurred nearby

The Formation of the Milky WayThe Magellanic Clouds are just two of the many small satellite galaxies of the Milky Way.

Many of these galaxies (including the LMC and the SMC) pay a price for being so close… they are torn apart by the gravity of our Galaxy. The stars and gas from the smaller galaxies become part of the Milky Way’s halo or the disk.

The Milky Way is a cannibal!

Astronomers now consider this process as the main way that large galaxies are formed.

The Formation of the Milky WayThe details of this process can now be captured in computer simulations that include:

• decreasing cosmic density due to expansion

• gravitational collapse of protogalactic clouds

• gravitational attraction between protogalaxies

• collision and merging of protogalaxies

• gravitational influence of dark matter(more on that subject later!)