More Topics in Astronomy OLLI at UNCA, Spring 2017 Mark Whipple

Week 4: The Milky Way

More Topics in Astronomy Course Outline 1: Eclipses 2: Moons of the Solar System 3: The Sun 4: The Milky Way 5: Dark Matter 6: The Big Bang 7: Dark Energy 8: The Size of the Universe

The Milky Way Named in ancient times for its appearance, a faint white band across the night sky, best seen during the summer and fall seasons. Photography reveals dark lanes that permeate white regions. It is actually composed of millions of stars, so far away that we see them only in bulk. Although it forms a band around the entire sky, one section of it is much brighter than the rest, located in the direction of the constellation Sagittarius.

William Herschel • • • • •

1738-1822

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Born in Germany, but lived most of his life in England Discovered (1781) the planet Uranus and several moons of Uranus and Saturn Discovered infrared radiation Began a catalog of nebulae that eventually became the New General Catalogue First to discover that the Sun and the Solar System are moving, and in approximately what direction Wrote 24 symphonies and numerous concertos, and…

…and he proposed that the Milky Way is a flat disk composed of a huge number of stars

This is the shape of the Milky Way as proposed by William Herschel from a careful count of stars; he incorrectly assumed that we are located at the center.

Immanuel Kant •

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Proposed that the Milky Way is a rotating disk held together by gravity, much like our Solar System Proposed the “nebular hypothesis” of the Solar System: that it began as a cloud of gas and dust that condensed into a sun and planets Had a few things to say about philosophy also if you’re into that

1724-1804

The galaxy as a universe Until about a century ago, it was assumed that our own Milky Way was the only galaxy, surrounded by infinite empty space. The low quality of images recorded by telescopes made it impossible to distinguish between other galaxies (top) and clouds of dust and gas in our own galaxy (bottom).

Cepheid variables The key to figuring out that there were other galaxies besides our own came from the properties of variable stars, specifically “Cepheid” variables. These stars have the property that their period of variation increases with their brightness—but that fact was unknown since their varying distances made it impossible to tell which ones were truly brighter than others.

Small and Large Magellanic Clouds

These Southern Hemisphere naked-eye objects were recognized as small “companion” galaxies to the Milky Way.

Henrietta Leavitt • Assigned as a “computer” by the Harvard Observatory to observe and measure the properties of variable stars, specifically Cepheids • She was unpaid at first, later got a raise to 30 cents per hour • Began observing Cepheids in the Magellanic Clouds, whose stars are all basically the same distance away • Discovered the relation between the period and the luminosity of Cepheid variables

1868-1921

Luminosity vs Period of variable stars

Globular clusters •

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Usually contain between 100,000 and 1 million stars Almost entirely Population II (older, first generation) stars, mostly red dwarfs Virtually no remaining gas and dust, all of it drawn in to stars long ago There are believed to be between 150 and 200 of them in the Milky Way

Harlow Shapley 1885-1972 Harlow Shapley mapped out the locations of the known globular clusters in the night sky. Since these are among the largest structures in the galaxy, and since they are heavily centered in one direction, he concluded that we are not near the center of the galaxy, but out towards the edge.

Shapley-Curtis debate 26 April 1920, Washington DC

Harlow Shapley Heber Curtis

Shapley argued (incorrectly) that spiral “nebulae” (other than the Magellanic Clouds) were objects located in the Milky Way galaxy, not other galaxies separate from our own. Telescopes at that time were unable to resolve individual stars in those distant objects.

Shapley argued that a nova seen in the Andromeda “Nebula” proved that it could not be a separate galaxy—the nova could not be so bright from such a distance. He was unaware that the nova was in fact a supernova, an unknown object at the time.

Why we need good science teachers in our schools

The Doppler Effect (sound waves)

Doppler shift: light waves

Doppler Effect: measuring stellar motion

Stellar motion

The motion of a star in the sky has two components: its radial motion towards or away from us (which can be measured easily and accurately by Doppler shifts), and its side-to-side motion, which is much more difficult.

Proper motion: Big Dipper

The stars in our galaxy are all moving, but ones close to each other usually are moving in a similar direction. This means that the constellations will stay more or less the same over extended periods of time—but only if the stars in them really are close to each other in space.

We can all probably use a break right about now.

Looking towards the center

The densest concentration of dust and nebulae in space is located in the direction of the constellation Sagittarius.

What does the Milky Way look like? The problem with “seeing” the Milky Way is that we are inside of it, the worst possible location for observing it as a whole. This is made even worse by the large amount of dust in the galaxy’s disk, which completely blocks our view of the center, much less what is on the other side.

So what does the Milky Way look like from the outside?

Sorry about that.

The worst possible place to see the Milky Way: from the inside of it

Patterns of star motion When we measure Doppler shifts for thousands of stars and look for overall patterns, we see that red and blue shifts tend to be divided into these quadrants.

Star motions in the Milky Way

Halo stars vs disk stars Stars in the galactic halo (a spherical region around the galaxy’s center, which includes the disk) are almost entirely Population II (older, composed of hydrogen and helium only) Disk stars are a mixture of Population II and Population I (second generation stars with some heavier elements)

Why would halo stars be different from stars in the galaxy’s disk? It has to be because of when they formed originally. Early in the galaxy’s history, when it was just a big blob of gas, stars could—and did—form anywhere. But the galaxy’s rotation caused it to flatten out, and all the free hydrogen gas drifted into the disk. There was no gas left in the halo to form new stars, so the only stars we see there are older ones (Population II).

Our place in the Milky Way

The Milky Way from several perspectives

A barred spiral! The Milky Way is between 100,000 and 180,000 light years across, about 2,000 light years thick, and we are located about 26,000 light years from the center. There are between 100 and 400 billion stars in the galaxy, and it takes our Solar System about 240 million years for each orbit around the center.

Magellanic Clouds

The Milky Way and its two “companion” galaxies

Which way to the center? The most powerful source of radio waves coming from any direction in space is a location in the constellation Sagittarius, and is called Sagittarius A. This is also right in the center of the brightest part of the Milky Way as seen from Earth.

At the center Located within the region of Sagittarius A, at the center of the Milky Way, is a spot known as Sagittarius A*. There is no visible object here, but the location has a mass that can be measured by the motion of nearby stars. This mass is now estimated to be about 4.3 million times the mass of the Sun.

A more recently updated image

A gas cloud approaches Sgr A*

2011

2012

2013

2014

2016?

The object known as G2 did not get ripped apart as expected, leaving open speculation that it was not really a gas cloud.

What is Sgr A*? The only explanation for Sgr A* that works is that there is a huge black hole at the center of the Milky Way surrounded by an accretion disk. As matter falls inside the event horizon, energy is released along the poles of this rotating monster. From studies of other galaxies, this is not at all unusual; most or all galaxies have supermassive black holes at their centers, many of them much larger than our own.

Next week: Dark Matter