Ch. 11: The Deaths and Remnants of Stars (part a) The evolution of intermediate-mass stars. Planetary nebulae and the formation of white dwarf stars. Supernova explosions: two types Type I: due to “carbon detonation” of an accreting white dwarf in a binary. Type II: due to “core collapse” in a high-mass star. Both types of supernovae leave behind remnants. Evidence from clusters confirms our theories of stellar evolution. Compact objects: neutron stars, pulsars, quark stars, and black holes.

In a young star on the main sequence, hydrogen shell burning occurs around an “ash” core, which is mostly helium. The core temperature is about T = 10 million K

Low-mass Stars - between 0.08 and 0.4 times the mass of the Sun have low core temperatures, live a long time, convect helium from the core, so it mixes uniformly, and will end up composed entirely of helium.

Stars with mass greater than 0.4 solar masses burn faster. During stage 7 hydrogen burning causes a build-up of helium in the star’s core. Eventually a core of helium “ash” accumulates in the core. On the next slide, we follow the evolution of a star like the Sun, with one solar mass.

Helium shell burning continues, and carbon burning commences

Examine each of these in detail on next 3 slides:

Planetary Nebulae form when the core can’t reach 600 million K, the minimum needed for carbon burning.

A Planetary Nebula shaped like a sphere, about 1.5 pc across. The white dwarf is in the center.

A Planetary Nebula with the shape of a ring, 0.5 pc across, called the “Ring Nebula”.

Cat’s Eye Nebula, 0.1 pc across, may be from a pair of binary stars that both shed envelopes.

M2-9 has twin lobes leaving the central star at 300 km/sec, reaching 0.5 pc end-to-end.

(See the slide show of planetary nebulae.) Many more examples of planetary nebulae are known: NOAO: AAO: ESO: And for a list of the Messier Catalog, see the SEDS Messier database:

Sirius Binary System: Sirius B is a white dwarf

White Dwarf formation on the H–R Diagram 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”

Sirius B has a high mass for a white dwarf, and probably came from a mass 4 M solar star.

Sirius B is at the 5 o’clock position.

X-ray picture of Sirius A 11,000K and Sirius B 24,000K

Evolutionary tracks: supergiants to white dwarfs

Distant White Dwarfs in Globular cluster M4.

A Nova is an explosion on a white dwarf, but only a small amount of material on the surface of the white dwarf explodes. Nova Herculis 1934 a) in March 1935 b) in May 1935, after brightening by a factor of 60,000

Nova Persei - matter ejection seen 50 years after the 1901 flash (it brightened by a factor of 40,000).

Nova light curve – due to a nuclear flash on a white dwarf

Supernova explosions There are two types of supernova explosions: Type I: due to “carbon detonation” of an accreting white dwarf in a binary star system. Type II: due to “core collapse” in a high-mass star, forming a neutron star or black hole. They have distinctive light curves (next slides).

Artist’s drawing of a Close Binary System

Type Ia supernovae When enough carbon accumulates on the close binary white dwarf, it can suddenly start carbon fusion, and with no outer layers, it will completely explode. This has about the same brightness for each explosion because it happens at a particular limit of the star’s mass (1.4 solar masses). These can therefore be used to estimate distances to remote galaxies (10 9 ly away).