Physics 101 - Astronomy - Spring 2019

Class notes for day 26, April 23, 2019

The Deaths and Remnants of Stars

Planetary Nebulae form when the core can’t reach 600 million K, the minimum needed for carbon burning. This happens for stars less than 8 times the mass of the Sun. Some heavier elements are formed in the last years of the burning in the shells surrounding the carbon core. H, He, C, O, and some Ne and Mg are expelled from the star as a “planetary nebula”. This is called a "planetary nebula" because early astronomers saw a spherical object that could be confused with a planet, but they are really expanding shells of gas from the old star.

Many images are available; here are some good links to images of planetary nebulae:

NOAO: http://www.noao.edu/image_gallery/ has a link to two pages of images of planetary nebulae:
http://www.noao.edu/image_gallery/planetary_nebulae.html
http://www.noao.edu/jacoby/pn_gallery.html
Hubble: http://hubblesite.org/images/news/34-planetary-nebulae
ESO: http://www.eso.org/public/images/archive/category/nebulae/ (not all of these are planetary nebulae)

Explosions of white dwarf and high-mass stars

High-Mass Evolutionary Tracks are quite different from the low-mass stellar evolution tracks.

A Type I Supernova is a “carbon detonation” and involves a white dwarf which completely explodes. As material accretes on the white dwarf from a binary companion, it’s mass finally reaches a critical limit, and the entire carbon core fuses to heavier elements, all at once.

A Type II Supernova is a “core collapse” and occurs when the core is finally pure iron, which cannot be fused to other elements. The core collapses to a big ball of neutrons, which causes a shock wave to bounce back outward, which blows off the entire envelope of the red giant, to form a large expanding nebula called a "supernova remnant." These look quite different than planetary nebula and contain much hotter gasses.

Historical supernovae in the Milky Way (none were observed by telescope): http://messier.seds.org/more/mw_sn.html
Latest supernovae (by current brightness !): http://www.rochesterastronomy.org/supernova.html
Supernova SN2005cs in M51 (Whirlpool galaxy): http://www.rochesterastronomy.org/sn2005/sn2005cs.html also see:
http://apod.nasa.gov/apod/ap050719.html

Supernovae remnants:

Cass A: http://apod.nasa.gov/apod/ap050615.html

http://apod.nasa.gov/apod/ap040826.html

N63A: http://apod.nasa.gov/apod/ap050608.html

SN1987A:

http://apod.nasa.gov/apod/ap990209.html

http://apod.nasa.gov/apod/ap980217.html

http://apod.nasa.gov/apod/ap000217.html

http://apod.nasa.gov/apod/ap000512.html

http://apod.nasa.gov/apod/ap120227.html

http://apod.nasa.gov/apod/ap120226.html

Eta Carinae will become a supernova in the next 100,000 years or so: http://messier.seds.org/xtra/ngc/etacar.html

Neutron Stars

In a Type II Supernova “core collapse” the process does not stop with the formation of a white dwarf. A more dense object can be created. A neutron star is a big ball of neutrons which is formed in some Type II explosions. A neutron star is a solid sphere, made of neutrons, about 20 km across, with a density over 10¹⁸ kg/m³ . The density is so high that a thimbleful of this material would have the mass of a small mountain on Earth.

Pulsar Radiation is believed to come from spinning neutron stars. About 1500 of these objects are known. They are created in the core collapse that causes the Type II Supernovae. The exterior of the star is blown off, and only the neutron star remains. It spins rapidly, from about once per second to 1000 times per second. A Millisecond Pulsar rotates very rapidly, after millions of years of spinning up due to accretion of incoming material from a binary companion. A current model says that pulsars are due to spinning neutron stars which are accreting gas at magnetic poles. The spin of the star causes the hot region to sweep by our direction like the light from a lighthouse, which explains the name “lighthouse” model. The Crab Nebula contains a pulsar. The Crab Pulsar is due to a spinning neutron star that rotates 30 times per second. This was created in a supernova which was seen in the year 1054.

X-Ray bursters are due to nuclear explosions on the surface of an accreting neutron star. This is different than the case of a nova, which was a nuclear explosion on the surface of an accreting white dwarf.

I showed a number of images of the Crab Nebula and its pulsar (spinning neutron star at the core). These were obtained mostly from the website for the Chandra orbiting X-ray observatory, which has images and movies of supernova remnants:

http://chandra.harvard.edu/photo/category/snr.html

Black holes are an extreme form of matter compressed into a point! The space around a massive object is "warped" by the effects of the large mass, and this can cause the deviation of light which travels past a massive object like a star. This effect has been observed in eclipses of the Sun, which causes the stars seen close to the Sun to be a different apparent position than they really are, due to the bending of light in the space near the Sun. Black holes probably form during supernova explosions, when the collapse of the core continues past the density of neutron stars. They have a huge amount of mass, like the mass of an entire star, and will attract nearby mass just like any other large mass. But any mass (or light!) falling past the event horizon is lost forever, and will never escape. Black holes are completely invisible, because light cannot escape. However, the accretion disk will be very hot, and will radiate large amounts of X-rays, UV, visible light, radio waves, etc. The Chandra Observatory has taken X-ray pictures of the center of the Milky Way Galaxy (our galaxy), which indicates a supermassive black hole at the center of the galaxy. This supermassive black hole has the mass of about 4 million solar masses.

Exam # 4 is this week, Thursday, April 25. The exam will have about 44 questions.

In the OpenStax online textbook, to prepare for the fourth exam, read these sections:
17.1-4, 18.1-4, 19.1-2, 20.1-6, 21.1-3, 22.1-5, and 23.1-4 (and skim Ch. 24).
Skip the biographical notes and other extra material in the boxes if you don’t have time.
Review the Key terms and Summary sections at the end of each chapter.