ASTR 1102-002 2008 Fall Semester

ASTR 1102-0022008 Fall Semester

Joel E. Tohline, Alumni ProfessorOffice: 247 Nicholson Hall

[Slides from Lecture14]

Chapter 20: Stellar Evolution:The Deaths of Stars

and

Chapter 21: Neutron Stars

Excerpt from §21-1

On the morning of July 4, 1054, Yang Wei-T’e (imperial astronomer to the Chinese court) made a startling discovery. Just a few minutes before sunrise, a new and dazzling object ascended above the eastern horizon.

This “guest star” was so brilliant that it could easily be seen during broad daylight for the rest of July!

This “guest star” was visible in the night sky (to the naked eye) for 21 months.

Crab Nebula

Today, if we look at the location on the sky where Yang Wei-T’e discovered his “guest star” nearly 1000 years ago, we see a glowing gaseous nebula that we call the “Crab Nebula”: – The gaseous debris is expanding away from

its center at a rapid rate; – projecting this expansion rate backward in

time, we conclude that the nebula originated from a “point-like explosion” approximately 1000 years ago

Crab Nebula

Crab Nebula

Today, if we look at the location on the sky where Yang Wei-T’e discovered his “guest star” nearly 1000 years ago, we see a glowing gaseous nebula that we call the “Crab Nebula”: – The gaseous debris is expanding away from its center

at a rapid rate;

– projecting this expansion rate backward in time, we conclude that the nebula originated from a “point-like explosion” approximately 1000 years ago

Crab Nebula

• At the center of the crab nebula, astronomers have identified a peculiar, compact star (a “pulsar”) that …– At visible wavelengths is difficult to see;– At radio wavelengths is a powerful “light-house”

beacon that flashes on and off 33 times every second!

Crab Nebula

• Astronomers are convinced that the gas making up the Crab Nebula is (what is left of) the outermost layers of a massive star that died violently (a “supernova explosion”) in the year 1054, and that its central pulsar is a rapidly rotating neutron star – a compact stellar remnant, which was once the “core” of the highly evolved, massive star.

• This illustrates how massive stars die!

Crab Nebula

• Astronomers are convinced that the gas making up the Crab Nebula is (what is left of) the outermost layers of a massive star that died violently (a “supernova explosion”) in the year 1054, and that its central pulsar is a rapidly rotating neutron star – a compact stellar remnant, which was once the “core” of the highly evolved, massive star.

• This illustrates how massive stars die! The “disaster” alluded to earlier results in an explosion of cataclysmic proportion.

Supernovae

• Easily (and now frequently) detected in other galaxies. (Statistically, every galaxy should display 1-3 supernovae every 100 yrs.)

• The light display from each SN generally can be categorized as one of several standard “types”:– Type Ia– Type Ib, Ic– Type II

What About SNe in Our Galaxy?• We’ve already discussed the Crab SN, which exploded

in 1054; our distance from the Crab nebula is about 2000 parsecs, and it is approximately 4 pc in diameter.

• Over the past 1000 years, written records indicate that only 5 SN explosions have been seen (by humans) in our “Milky Way” Galaxy– Years 1006, 1054 (Crab), 1181, 1572, 1604

• We’re overdue!• NOTE:

– Dozens of gaseous SN remnants are identifiable in our Galaxy– Well over a thousand (!) pulsars have been catalogued in our

Galaxy.

SN 1987A

• On February 23, 1987, a supernova was discovered in the Large Magellanic Cloud (LMC), a companion galaxy to our Milky Way some 51,500 pc (168,000 ly) from Earth.

• This supernova (SN 1987A) could easily been seen (from southern hemisphere) at night w/o aid of telescope; but was not bright enough to be seen in the daytime.

• Given the available array of modern telescopes and detectors, SN 1987A has gave us an unprecedented “laboratory” for understanding supernova explosions.

Andromeda Galaxy

Magellanic Clouds

Magellanic CloudsSMC

LMC

SN 1987A




Large Magellanic Cloud (LMC)

SN 1987 A

SN 1987A




SN 1987 A

Neutrinos from SN 1987A• Two “particle physics” detectors were in

operation (deep underground) on February 23, 1987.– Kamiokande detector (Japan)– IMB (Irvine, Michigan, Brookhaven) detector (U.S.)

• A 12-second “burst” of neutrinos was detected (12 by Kamiokande; 8 by IMB) 3 hours before astronomers saw light from the exploding star.

• Actual equivalent of exposure to a “torent of more than 1016 neutrinos.”– 100 times the energy the Sun has emitted during its

entire history!– 100 times that the supernova (1987A) emitted as

light!

Discovery of Pulsars(neutron stars)

ASTR 1102-002 2008 Fall Semester

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