Outline - March 29, 2010

• Stellar Remnants: white dwarf, neutron star, pulsar, black hole (pgs. 588-589, 592-594, 595-600)

• 2 types of supernovae (pgs. 590-591)

• Hypernovae and gamma ray bursts (pgs. 601-603)

• The Milky Way - basic structure, stellar populations, stellar motions (pgs. 611-615)

• Star formation in the Milky Way (pgs. 623-625)

• Formation of the Milky Way (pgs. 626-628)

Stellar “Remnants”What’s left behind after a star dies?

Main sequence mass < 5 Msun: white dwarf

Main sequence mass between 5 Msun and 40 Msun: neutron star

Main sequence mass > 40 Msun: black hole

All of these are stable (neither expanding nor contracting), so long as they are “left alone”.

Pressure in white dwarf and neutron star is somewhat exotic (not normal gas pressure or radiation pressure) due to their

highly-compressed states.

White Dwarfs in Binary Systems

• Most stars are found in binary systems

• May have situation where WD orbits a giant or supergiant star at a relatively close distance

• Outer layers of the giant or supergiant are very light and fluffy, and may be pulled over onto the WD by gravity

• Material from companion star builds up in an “accretion disk” around the WD, and eventually winds up on the surface of the WD

White Dwarfs in Binary Systems, II

What happens to the WD when mass is dumped onto it depends on how much mass, and how fast.

Slow accretion of not much mass (not enough to make the mass of the WD > 1.4 Msun): nova

Fast accretion of a lot of mass (enough to make the mass of the WD > 1.4 Msun rather suddenly): supernova (“Type 1a”)

Novae

• Thin layer of (mostly) H from the companion star builds up on surface of WD

• Sudden flare in brightness (increases by about a factor of 10,000 or more), then fades over the course of about a month

• Flare is due to hydrogen fusion on the surface of the white dwarf

• Novae happen about 2 or 3 times per year in our Galaxy

• Can recur (i.e., same WD can “go nova”, but not very predicable)

NovaeH fusion on the surface of a WD

“Naked eye” nova; picture taken in the Varzaneh Desert in Isfahan, Iran (February 2007)

White Dwarf SupernovaSupernova Type Ia

If the companion star to a WD dumps a lot of mass onto the WD very quickly, making the mass of the WD exceed the Chandrasekhar mass (1.4 Msun), the WD explodes as a

supernova!

WD is much like a hot metal ball, same temperature and same density throughout

Addition of extra mass causes WD to contract (gravity “wins” over pressure from the electrons) and instantaneously the carbon starts to fuse throughout all parts of the WD,

blowing the WD to bits

Two Basic Types of SupernovaeNote: Supernovae NEVER repeat!

Remnants of two different supernovae. Left: a Type Ia supernova (WD). Right: a Type II supernova (high mass star). This is a “happy alignment” of images - the two stars weren’t related to each other!

What’s left behind after a massive star goes supernova?

If the mass of the core is less than about 3 Msun, a neutron star is left behind.

If the mass of the core is greater than about 3 Msun, there is no source of sufficient pressure to keep the core from collapsing completely under gravity, and a black hole is formed.

Neutron Stars

• Even more compressed than WD

• Typically the size of a city (about 10 km in radius) with mass between 1.4 Msun and 3 Msun

• Density is such that the weight of one teaspoon of NS material would weigh 100 million tons (vs. 1 ton for WD material)

• NS supported by “neutron degenerate pressure” (again, quantum mechanical phenomenon having to do with how tightly neutrons can be packed together)

• Must rotate extremely fast (conservation of angular momentum); a star that was originally rotating once per month would now have to rotate a few times per second!

• Compression of the material also compresses the magnetic field, and amplifies its strength (making it trillions of times larger than the earth’s magnetic field)

PulsarsRapidly-Rotating Neutron Stars

First discovered by Jocelyn Bell (1967) as “pulses” of radio light coming from the Crab Nebula

The pulses lasted 0.01 seconds, and repeated every 1.4 seconds

In 1974, Jocelyn’s PhD advisor (Tony Hewish) got the Nobel Prize for explaining what puslars are

Now know of 100’s of pulsars, most with periods between 0.03 and 0.3 seconds (meaning they rotate between 3 and 30 times per seconds)

The fastest known pulsars have periods of milliseconds and are rotating at speeds approaching 0.25c !!!!

The radio light from plusars is called “synchrotron radiation”, a type of light that is emitted by electrons as they move on spiral paths around magnetic field lines. The “synchrotron radiation” pulses are proof of the fast rotation rates of neutron stars and the presence of an incredibly strong magnetic field.

Pulsar at Center of Crab Nebula

Supernova observed by Chinese astronomers in year 1054

Crab Nebula - remnant of supernova explosion

Pulsar Recordingshttp://www.astrosurf.com/luxorion/audiofiles-pulsar.htm

Crab - 1.4 rotations per second Vela - 11 rotations per second

PSR 1937+21 (a “millisecond” pulsar) - 642 rotations per second

Is there anything bigger than a supernova?(Maybe)

Curious events discovered serendipitously: “gamma ray bursts”

Gamma rays are VERY high energy light, inherently rare for them to be emitted by astronomical objects

Bursts discovered in early 1960’s, randomly distributed around the sky, a few per week, may have complex light curves, but they don’t repeat

Origin (inside the Milky Way vs. outside the Milky Way) determined in 1997: are “extragalactic” in origin, but what’s the cause?

Gamma-ray Bursts

Gamma-ray bursts are all found in galaxies other than the Milky Way and sometimes they have “optical afterglows”.

Origin of “long” -ray bursts:

Explosion of star with mass > 40 Msun (core becomes a black hole)

Now often called a “hypernova” (more energy than a typical high-mass star supernova)

Newly formed black hole in core sucks in some of mass surrounding core during few seconds while star explodes

Infalling gas forms disk and a jet of material and radiation

Flash (few seconds) of rays: gamma-ray burst

X-ray & visible light “afterglow” lasts for hours or days

Where do we go from here?

We’ve spent the last several classes looking at a lot of details of stars, star formation, star evolution, and star death.

Now we’re going to go for the “big picture”. Where do all of these things tend to be found - is there a pattern and, if so, what can it tell us?

We’ll start at home: The Milky Way Galaxy

What’s a galaxy?

• Always a vast collection of stars (100’s of millions to 100’s of billions)

• Often galaxies also contain dust and gas (the ISM)

• Milky Way is our home galaxy, and is a very typical spiral galaxy

• Every individual star that you can see with the naked eye from the is a member of our Milky Way galaxy

Visualizing the Milky Way Galaxy

This is tough to do! We’re living inside the galaxy, and we can’t simply send a satellite outside the galaxy to send a picture back.

Instead, we have to carefully study the stars, gas, and dust, then piece together the whole picture.

Note: the sun is nowhere near the center of the galaxy

Three major components to the Milky Way: bulge, disk, and halo (only see two of these in the drawing above).

“Edge-on” vs. “Face-on” Orientations

Face-on

Edge-on

Most of the stars (and gas and dust) reside in a thin circular disk

Aspect ratio (thickness/radius) of the disk is comparable to a CD (about 100:1)

In the edge-on view, stars in the halo are visible. The halo stars form a roughly spherical distribution.

The disk is embedded within the halo (and halo stars will often pass through the disk of the galaxy)

Which stellar components (disk, bulge, halo) do you see in what part of the sky?

Scale Size of the Milky Way

Diameter of disk: 100,000 light years

Thickness of disk: 1,000 light years

Diameter of bulge: 18,000 light years

Radius of sun’s orbit: 28,000 light years

How do stars orbit in the Milky Way?

Bulge stars and halo stars have randomly-inclined elliptical orbits about the center of the galaxy.

Disk stars have nearly-circular orbits, all in the same direction, about the center of the galaxy.

Bulge and halo: random motion

Disk: ordered motion

Note: bulge and halo stars will (and do!) pass through the disk on their orbits

Measuring the Mass of the Milky Way

The total mass of the MW that is contained within a radius of r is given by:

Mr = (r v2) / G

where v is the speed that stars at radius r orbit and G is Newton’s constant

For the sun’s orbital radius (28,000 light years) and orbital speed (220 km/s), we find a mass of 1.0x1041 kg, or about 100 billion Msun.

This mass is much too large to be explained by the sum total of stars, gas and dust in the Milky Way - it is evidence for “dark matter” in our galaxy.

Distribution of Material in the Milky Way: Disk

Contents of disk: old stars, young stars, cold gas, hot gas, dust

Region of active star formation: spiral arms (location of virtually all the gas)

Optical color: very blue (due to presence of young O and B stars)

Spiral galaxy M81 showing star light (left) and cold H gas (right). There is much more gas in the arms than anywhere else. There are not vastly more stars in the spiral arms, though. Preferentially the brightest, bluest stars are found in the arms.

Hot Gas in the Disk

Hot, young stars in the spiral arms heat up the gas around them (above) forming H-II regions. Other examples of hot gas that we’ve seen are planetary nebulae and supernova remnants (which can be seen both in the disk and outside the disk). H-II regions are associated with active star formation; planetary nebulae and supernova remnants are associated with star death.

H-II Regions in the Whirlpool Galaxy

The H-II regions are the pink dots that beautifully trace the spiral arms.

Distribution of Material in the Milky Way: Bulge

Contents of the bulge: high density of gas and stars, old and young stars

Active region of star formation (but can’t see at all well with visible light because of dust obscuration)

Optical color: yellowish

M31 (the Andromeda Galaxy), our “sister” galaxy

Distribution of Material in the Milky Way: Halo

Contents of the halo: only old stars, very little cold gas, globular star clusters

Optical color: very red

Putting it all together…

What does having a disk where active star formation is taking place, embedded within a spherical halo of old stars, tell us?

It’s all about how the Milky Way formed (and we see the some of the same sorts of things as when we talked about star formation).

Note: the oldest stars in the MW are the ones that formed first (and many are still around today - it’s only O and B stars that live fast and die young)

Formation of a Spiral Galaxy

What makes the spiral pattern?