Download Lecture 17, PPT version

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)
Novae
H fusion on the surface of a WD
“Naked eye” nova; picture taken
in the Varzaneh Desert in
Isfahan, Iran (February 2007)
White Dwarf Supernova
Supernova 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 Supernovae
Note: 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)
Pulsars
Rapidly-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 Recordings
http://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
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)
Edge-on
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?