Chapter 21: The Milky Way

William Herschel’s map of the Milky Way based on star counts

In the early 1800’s William Herschel, the man who discovered the planet Uranus, tried to understand the shape of the Milky Way by counting stars. He counted how many stars there were in each of dozens of different directions and then plotted the count versus direction.

Harlow Shapley looked at RR Lyrae stars in globular

clustersBy the early 1900’s a young astronomer named Harlow Shapley (the same one of the Shapley-Curtis debate on the spiral nebulae) tried to determine the size of the Milky Way by observing RR Lyrae stare in globular clusters.

RR Lyrae stars are variable stars with periods less than a day

Like Cepheid variables, RR Lyrae variables can be used to find distance by measuring their pulsation period and using the period-luminosity relationship.

Shapley’s view versus Herschel’s view

Herschel had tried to determine the shape of the Milky Way by counting the stars around us. Shapley wasn’t trying to find the shape of the galaxy, just how big it is and where its center is. He found the galaxy was about 100,000 lightyears across and the center was almost 30,000 lightyears away from us in the direction of the constellation of Sagittarius.

Modern Views ofthe Milky Way

IRAS

COBE

Compton

Edge on view of a spiral galaxy

One of the ways we can learn about our Milky Way is to study other galaxies. According to the cosmological principle, we are not in a special place. That means that our Milky Way isn’t any special kind of galaxy. It must look like other galaxies do so if we study them we might be able to learn about ourselves.

The View at 21 centimeters

One of the wavelengths that will be particularly useful is 21-cm. This is a radio wavelength that is able to penetrate through dust easily. It will allow us to observe all the areas in our Milky Way

The redshift or blueshift of the 21 cm line gives us structural

details of the Milky WayOne of the things we can do with 21-cm observations are Doppler shift studies.

The part of the Milky way we “see” is about 100,000 Ly

across

The visible matter in the Milky Way is about 100,000 lightyears across. We are located about 27,000 lightyears from the center. The galaxy appears to have a small central bar, possibly four main arms and a few spurs (arm fragments) although the most recent observations indicate that there may only be two arms, each wrapped around the center more than once. Two arms would be typical of a barred spiral.

The Milky Way is an SBbc barred spiral galaxy

The bar of our Milky Way has only been discovered in the last decade. Most barred spirals have two main arms coming off the ends of the bar but, as can be seen in the galaxy at right, that isn’t always the case. The number of arms the Milky Way has is still a topic of active research.

The rotation velocity of stars tells us there is a lot of Dark

Matter in the Milky Way

The Dark Matter, along with globular clusters and old

stars, form a halo around the galaxy

The dark matter halo extends our at least 150,000 lightyears from the center of the galaxy while the visible part only goes out 50,000 lightyears.

The Milky Way seems to have two stellar age groups

Globular clusters, found in the halo, are the oldest stars. The youngest globular cluster is over 8 billion years old while the oldest is over 12 billion years old.

Open clusters and the stars that have drifted out of them are found in the disk and are the young stars. The oldest open clusters are less than 5 billion years old.

Astronomers use the “metal” content of stars to find their

age

The history of star formation in the Milky Way

The Milky Way seems to have undergone a number of star forming periods followed by periods

of relatively little star formation.

Even the disk has different stellar age groups

Because of the different episodes of star formation, there are stars of many different ages in the disk of the galaxy.

Cosmic rays give us a picture of the galactic magnetic field

Cosmic rays are high energy particles, mostly protons, that are accelerated to speeds close to the speed of light in

supernova explosions and near black holes.

We see the galactic magnetic field is tied to the

gas clouds in the disk

Cosmic ray studies indicate that the galactic magnetic field is not uniform. It tends to be concentrated where there are giant molecular clouds and spread out and weaker in the region between the spiral arms.

The stars in the disk all orbit in the same direction

If we look at how the stars orbit around the galaxy we find they all orbit in the same direction. They tend to bob up

and down around the central thin disk as they orbit.

The stars in the halo orbit in random directions

If we look at how the stars and globular clusters in the halo orbit, we find an entirely different picture. The halo objects orbit in random directions and many of their orbits are very elliptical.

The very center of the Milky Way is

a very crowded

placeAt the very center of the galaxy, in a space only a few hundred lightyears across, we find a very crowded place.

A close-up of the very center of the Milky Way

Things move fast around the center

Watch Galaxy Center Star Orbits video

While Sagittarius A* isn’t moving, the things around it most definitely are, and fast

Sagittarius A* is a 4 million solar mass black hole

When we do the Kepler’s 3rd law calculation we find that Sgr A* is four million times the mass of the Sun.

At present it doesn’t seem to be eating much but it does occasionally have a snack. Watch Sgr A Star Flare

video

While it may not be eating now, the supermassive black hole at the center of the Milky Way has

eaten in the past.

Images in x-ray and gamma ray indicate a pair of bubbles and jets above and below the disk of the galaxy.

Understanding the formation of the Milky Way helps us

understand galactic formation

The Milky Way doesn’t live in isolation