The Deaths of Stars

The Southern Crab Nebula (He2-104), a planetary nebula (left), andthe Crab Nebula (M1; right), a supernova remnant.

Once the giant phaseof a medium-mass star ends,it exhales itsouter layers,making aplanetarynebula. Thedegeneratecore is a whitedwarf star.

In 1844 F. W. Bessel wasinvestigating the propermotion and parallax ofSirius. He predicted thatit had a faint, unseencompanion.

In 1862 Alvan GrahamClark discovered thisfaint companion. It wasthe first known whitedwarf star.

Sirius A and B at optical wavelengths(top left), and in X-rays (left).

In 1930 the young Indianphysicist SubrahmanyanChandrasekhar (1910-1995)discovered that a white dwarfstar can have a mass no greaterthan 1.4 solar masses.

A white dwarf star is comparable in size to theEarth. More massive white dwarfs are smallerthan less massive white dwarfs.

Three routes to an end state:

less than 0.4 solar masses – because they are fully convective, they use up all their mass slowly converting H to He. Do not become red giants.

0.4 to 8 solar masses – lose mass due to stellar winds during giant phase, exhale atmospheres (which become planetary nebulae), leave white dwarf remnant at center of planetary nebula

8 or more solar masses – explode as Type II supernovae, leaving behind black holes, neutron stars, or pulsars

Somehow orother, intermediatemass stars “know”they must shedsufficient mass tohave less than1.4 solar masses.

Ferrario et al. (2005)

Ring Nebula in Lyra (M 57). Note white dwarf in center.

Three lovelyplanetarynebulae.

More planetary nebulae.

What have we learned?

The final end state of a single star with less than 8solar masses is:

a)red giant starb)white dwarf starc)Type Ia supernovad)Type II supernova

The final end state for a star with more than 8solar masses is:

a)supergiant starb)white dwarf starc)Type Ia supernovad)neutron star or black hole

Close binary stars have different evolution than singlestars. Mass can be passed from one star to the other.

As the more massivestar swells to becomea red giant, a lot ofmass can be transferredto the formerly lessmassive star. Ironically,the originally less massivestar can be the one toreach its end state first.

The star Algol( Persei, markedwith pink arrow)is an eclipsing binary in which theoriginal less massivestar has evolved tobecome a red giant,which the originalmore massive staris still on the mainsequence.

Material from the accretion disk in a nova (or dwarf nova)settles onto the white dwarf. This can lead to periodicexplosions in the system.

SS Cygni is a dwarfnova that has an outburst just aboutevery 52 days.In the upper imageat left it is in its faintstage. In the lowerimage at left it is havingan outburst.

Stars with 8 or more solar masses end up with a many-layered structure, eventually with an iron core.

The Crab Nebula in Taurus is a SN remnant ofan object visible in the year 1054.

Iron is the mostlytightly bound nucleus.Nuclear reactions toform heavier atomswould use up moreenergy than they wouldproduce. The outerlayers squeeze downonto the iron core andthe star explodes as aType II supernova.

SN 1987A was the explosion of blue supergiantstar in the Large Magellanic Cloud, a satellitegalaxy of our Milky Way.

Neutrinos from this explosion were detectedon the Earth.

As the ejecta of the SN plow into the interstellarmedium, an expanding ring of shocked gas isobserved.

Spectrum of the Type II SN 2003hn. At least fouremission lines due to atomic hydrogen are visible.

Infrared andoptical lightcurves of theType II-PSN 2003hn.

We can findmany supernovaremnants in ourGalaxy.

There are two basic ways to make a supernova:

1) explosion of a single massive star

2) mass transfer to a white dwarf star (If the mass of the WD approaches 1.4 solar masses, the star explodes.)

Supernovae with hydrogen emission in theirspectra are called Type II. They are explosions ofsingle, massive stars.

Supernovae without hydrogen emission, but withsilicon absorption are Type Ia SNe They areexplosions of C-O white dwarf stars.

Note how similar the spectra of these two Type Iasupernovae are!

Type Ia supernovaehave very similarlight curves in theB-band and V-band.But the objects thatare brighter atmaximum lighthave light curvesthat decline moreslowly.

Optical lightcurves of theType Iasupernova2004S

The “decline rate”is the number ofmagnitudes that aType Ia supernovagets fainter in theblue band over thefirst 15 days aftermaximum brightness.Faster decliners areless luminous. Theobjects are standard-izable candles.

The absolutemagnitude atmaximum ofa Type Ia SNis correlated with the numberof magnitudes the SN declinesin the first 15days after maximumlight. Slow declinersare more luminousat maximum thanfast decliners.

Peak brightnessof Type Ia SNein the near-IR.They are nearlystandard candles.Only the fastdecliners thatpeak late arefainter.SN 2009dc is anoddball that mayhave producedmore than 1.4Msun of 56Ni.

The top 4 SNeshown here areintrinsicallybrighter than thebottom two.The top 4 peakin the near-IRa couple daysprior to the timeof B-band max.Note the differentstrengths of the2ndary maxima.

SN 1994D

Because Type Ia supernovae are so bright (4 billion timesbrighter than the Sun!) and because we can determine theirabsolute magnitudes, we can use them to determine distancesto galaxies halfway across the visible universe.

What would happen to the orbit of the Earth if the Sun somehow became a black hole?a.The Earth’s orbit would be unchanged.b.The Earth would sucked into the black hole.c.The Earth would be ejected from the solar system.d.We have no idea.