Red Stars, Blue Stars, Old Stars, New Stars Session 4

Julie Lutz

University of Washington

Stellar Evolution “Finales”

• From formation on, the evolutionary patterns of stars have depended strongly on mass, and the same goes for the final stages of evolution.

• Stars do lose mass as they go from the main sequence through other stages.

• Recall that the low mass stars are by far the most common.

H-R Diagram, 1 Msun

For the Lower Mass Stars--about 1 to 8 Solar Masses

• The star gets to the point where it has a carbon core.

• Core collapses but not hot enough to initiate carbon to oxygen fusion.

• Most of star’s mass collapses to “degenerate matter” and star becomes a white dwarf.

• Outer layers escape in a “planetary nebula”.

Low Mass Stars: Planetary Nebulae

• Nothing to do with planets!

• First one discovered by Sir William Herschel who discovered Uranus in 1781 looked greenish like the planet.

NGC6720

StarfishNebula

RoundPNe

NGC 3132

IC 418

IC 4406

Menzel 3

He 2-104

H-R Diagram, 1 Msun

What Happens after the PN?

• Star settles down in the white dwarf configuration.

• No more thermonuclear reactions.

Characteristics of White Dwarfs

• Matter in WD is “degenerate”. Atoms packed so tightly that electrons move freely between atomic nuclei.

• Densities are about a billion particles per cubic centimeter.

• The more massive a white dwarf, the SMALLER it is.

A Teaspoon of WD Material Would Weigh as Much as…

….a Large Cruise Ship

Stellar Old Age

• White dwarf stars-up to 1.4 solar masses

• Neutron star-1.4 to about 3 solar masses

• Black hole-greater than 3 solar masses

White Dwarf Stars

Sirius B

• Sirius A is brightest star in night sky, a main sequence A-type star (T=10,000K)

• Sirius B is about 1 solar mass but has a size about that of the Earth.

• T = 25,000K

40 Eridani B

• 0.5 solar masses• T= 16,500 K• 1/70 solar radius• 1.5xradius of Earth• Part of a triple star

system• Home system of

Spock of Star Trek

Characteristics of White Dwarfs

• Maximum mass 1.4 solar masses• Those less than 0.5 solar masses are He• More massive carbon and oxygen• Densities 10,000,000-1,000,000,000 gm/cc• Cooling times 10,000,000,000,000,000 yrs• Degenerate matter• Less massive = bigger size

Structure of a C/O White Dwarf

• Degenerate matter until just a few meters of the outer part--that’s normal matter, so the white dwarf does radiate according to its surface temp

• 70,000-5000 K

Why Are White Dwarfs No More than 1.4 Solar Masses?

• The gas law obeyed by degenerate matter is such that the more mass, the smaller in radius.

• Becomes a point source at 1.4 solar masses.

How about Old Stars with > 1.4 Solar Mass?

• Will get further than oxygen in the thermonuclear reactions in core.

• When collapse of core comes, electrons will be forced into atomic nuclei where they will combine with protons. This produces neutrons.

• Core of star becomes neutron star or a black hole

Stars with Masses More that 8x Solar on the Main Sequence

• Lose a lot of mass as they evolve off the main sequence. More mass=more mass loss.

• But they still can’t squeeze into that 1.44 solar mass limit to become a white dwarf as they approach the end of their nucleosynthesis.

• The more massive, the closer they get to an iron core towards the end.

Characteristics of Neutron Stars

• Mass range 1.44-3 solar masses

• Densities 100,000,000,000,000 gm/cc

• Size-few km

• Predicted mathematically in 1930s

• First observed in 1967--accidental discovery with radio telescope

What’s Beyond Degenerate Matter?

• Suppose the energy conditions are sufficient to force protons and electrons together to form neutrons?

• Star would be a ball of neutrons (perhaps with a thin skin of regular matter.

• Size: few kilometers diameter.• Neutron stars predicted mathematically in

1930s.

Rapidly varying radio sources

• Periods of seconds or less

• Binary?? No, too short• Pulsation?? No, too

hard to move the matter that fast

• Rapid rotation?

First Pulsar: Period = 1.337 seconds

Crab Nebula

What was known about the Crab Nebula in 1967

• It is the remnant of a supernova that exploded in 1054 AD (a naked eye object)

• The gas/dust in the nebula is expanding with velocities of 1000s of km/sec

• Exhibits a special radiation called “synchrotron”

• Star at center has no features in spectrum

Crab Nebula Neutron Star

• Observed pulsations in radio waves 33 times a second.

• Pulsations occur at all wavelengths--optical, X-ray, etc.

• What could it be?

Crab Nebula Pulsar in X-rays

Pulsar

• Rapidly rotating neutron star

• “Beaming” of radiation due to very strong magnetic field

• Few kilometers in size so it can rotate very rapidly

Pulsars

• About 1000 discovered

• Periods of milliseconds to minutes

• Some found inside supernova remnants, many not

• Nobel Prize 1974

Supernovas

• Final explosion of star which had about 10 solar masses or more when it was on the main sequence

• Rare

• Star gets iron core and then core implodes

• Outer layers lost--heavy elements created

• Core becomes neutron star or black hole

The Veil Nebula

The Gum Nebula

Cas A in X-rays

Youngest SNR known in Milky Way--150 years

Supernova 1987a

• Observed Jan 1987 in the Large Magellanic Cloud

• Became first magnitude star

• Visible with naked eye for about 2 months

For the Most Massive Stars

• May arrive at the “iron core” stage with more than 3 solar masses.

• Can’t make a neutron star with mass more than 3 solar masses.

• What comes next?

Black Holes Are Out of Sight!

• Most massive stars may form black holes

• Gravitation so strong that no radiation can escape

• How can we study black holes if we can’t see them?

• Binary systems with one black hole and one normal star

Black Holes Have Event Horizons

Bending of Light, Distortion of Space-Time

The Black Hole’s Gravitation Warps Space-Time

Black Holes

• What used to be the stellar mass resides in the singularity.

• Don’t know much about the state of that matter except that it has gravitation.

• Use General Relativity to deal.

Black Holes as Giant Vaccuum Cleaners

• If the sun were to suddenly become a black hole, nothing would happen to the Earth’s orbit.

• Mass would have to be within 10 miles of black hole sun in order to be sucked in.

Do stellar black holes exist?

• SS433--first noticed as X-ray source with periodic variations

• Normal star B-type• Companion is too

massive to be in the neutron star range

Black Hole Candidates in Binary Star Systems Name Companion Period Mass BH

Cygnus X-1 B supergiant 5.6 6-15LMC X-3 B main seq 1.7 4-11A0620-00 K main seq 7.8 4-9G (V404 Cyg) K main sequence 6.5 > 6GS2000+25 (QZ Vul) K main sequence 0.35 5-14GS1124-683 K main sequence 0.43 4-6GRO J1655-40 F main sequence 2.4 4-5H1705-250 K main sequence 0.52 > 4

Massive Black Holes Are found in the Center of Many Galaxies

X-Ray Milky Way Center, 2-3 Million Solar Mass Black Hole

With Supernova Remnants Often Don’t Know Stellar Result

• Could be a neutron star or a black hole.

• Can make a black hole at all masses.

• Picture shows remnant of 1006 supernova.

Bottom Line

• Black holes, neutron stars and white dwarfs are all known to exist

• Lots of work remains to be done in all areas of stellar evolution. Broad understanding, but details can confound.