1. Chapter 12 Predicting the Violent End of the Largest Stars

2. Mass Loss from a Supergiant Star Supernovae lose mass at a rapid rate in a strong stellar wind. As this wind collides with the surrounding interstellar gas and dust, it creates a “bubble.”

3. The Structure of an Old High-Mass Star At the heart of this nebulosity, called SMC N76, lies a supergiant star with a mass of at least 18 M. This star is losing mass at a rapid rate in a strong stellar wind. As this wind collides with the surrounding interstellar gas and dust, it creates the “bubble” shown here. SMC N76, which has an angular diameter of 130 arcsec, lies within the Small Magellanic Cloud, a small galaxy that orbits our Milky Way. It is about 60,600 pc (198,000 ly) distant.

4. A Core-Collapse Supernova

5. Luminous Supernovae Maximum luminosity as great as 10 9 L, rivaling the light output of an entire galaxy for a brief period. It’s possible to see supernovae in other galaxies. The Tarantula Nebula is 51,000 pc from Earth, but was bright enough to be seen without a telescope.

6. A Supernova in a Distant Galaxy The progenitor star that later exploded into SN 1993J was a K0 red supergiant. SN 1993J resulted from the core collapse and subsequent explosion of a massive star.

7. An Unusual Supernova SN 1987A appears to have a set of three glowing rings Relics of a hydrogen-rich outer atmosphere, ejected by gentle stellar winds from the star when it was a red supergiant. The gas expanded in a hourglass shape because it was blocked from expanding around the star’s equator either by a preexisting ring of gas or by the orbit of an as-yet- unseen companion star. These rings were ionized by the initial flash of ultraviolet radiation from the supernova.

8. Supernovae Remanants As this expanding shell of gas plows through space, it collides with atoms in the interstellar medium, exciting the gas and making it glow. The passage of a supernova remnant through the interstellar medium can trigger the formation of new stars. The death of a single massive star can cause a host of new stars to be born.

9. Our Supernova Neighbor The Gum Nebula exploded around 9000 B.C. At maximum brilliance, the exploding star probably was as bright as the Moon at first quarter. Like the first-quarter moon, it would have been visible in the daytime!

10. Seeing Supernovae Many supernova remnants are virtually invisible at the visible wavelengths our human eyes can see. However, when the expanding gases collide with the interstellar medium, they emit energy at a wide range of wavelengths, from X rays through radio waves. This is a radio image of the supernova remnant Cassiopeia A. As a rule, radio searches for supernova remnants are more fruitful than visible-light searches.

11. Supernovae of the Past The cores left behind by a supernova event are not easily visible to the human eye. Historically, supernovae were recorded as “guest stars,” giving us clues as to where old cores may be found. Some of these cores are neutron stars, or pulsars.

12. Pulsing Stellar Remnants In the summer of 1967, Jocelyn Bell and colleagues picked up a pulsing radio light signal. These objects pulse incredibly fast. An object that changes so quickly must be quite small. The only object that could be small enough to spin at such a high rate and not fling apart is a highly compact, high-density, tiny neutron star. Indeed, pulsars are rapidly rotating neutron stars with intense magnetic fields.

13. A Rotating, Magnetized Neutron Star Charged particles are accelerated near a magnetized neutron star’s magnetic poles, producing two oppositely directed beams of radiation. If the star’s magnetic axis is tilted at an angle from the axis of rotation, the beams sweep around the sky as the star rotates. If the Earth happens to lie in the path of one of the beams, we detect radiation that appears to pulse on and off.

14. The Formation of a Black Hole When the star becomes a black hole not even photons emitted directly upward from the surface can escape; they undergo an infinite gravitational redshift and disappear.

15. The Formation of a Black Hole Neither light, nor anything else, is able to escape the gravitational attraction of the crushed core. An object from which neither matter nor light (electromagnetic radiation) can escape is called a black hole. In a sense, a hole is punched in the fabric of the universe, and the dying star seems to disappears into this cavity. None of the star’s mass is lost when it collapses to form a black hole, and a black hole’s gravitational influence can still be felt by other objects.

16. Non-rotating Black Holes Once a non-rotating star has contracted inside its event horizon, nothing can prevent its complete collapse. The star’s entire mass is crushed to zero volume—infinitely dense—at a single point, known as the singularity, at the center of the black hole. The black hole has only two parts: a singularity at the center surrounded by a spherical event horizon.

17. Finding Black Holes Because light cannot escape from inside the black hole, you cannot observe one directly, in the same way that you can observe a star or a planet. The best you can hope for is to detect the effects of a black hole’s powerful gravity.

18. The Effects of Black Holes In binary systems, the mass from one star may be “captured” by the black hole. This material spirals into the black hole with so much speed that it emits X-ray light.

19. The Environment of an Accreting Black Hole If a black hole is rotating, it can generate strong electric and magnetic fields in its immediate vicinity. These fields draw material from the accretion disk around the black hole and accelerate it into oppositely directed jets along the black hole’s rotation axis.

20. Detonating a White Dwarf: Another Kind of Supernova

21. Telling the Supernovae Apart Type II Supernovae: Massive stars collapsing due to gravitational energy  A one-time event, releasing metals and emitting a hydrogen spectra Type I Supernovae: White dwarfs exploding 100% of their mass into space  A one-time event, releasing oxygen, carbon, and silicon, with almost no hydrogen spectrum

22. Novae Novae are NOT small supernovae. They are exotic objects: faint stars that brighten suddenly, then dim over a period of months. The result of a binary system in which a stellar corpse is gravitationally bound to another star.

23. Binary Systems Produce Variable Spectra

24. Accreting White Dwarfs: Novae vs. Supernovae Type I Like in a Supernovae Type I, material accretes, but not enough to cause nuclear reactions inside of the white dwarf. In a nova, there is only enough material to cause fusion on the surface. The white dwarf survives the explosion, allowing the nova to reoccur.

25. X-ray Bursters and Neutron Stars Neutron stars also occur in binary systems. They can accrete material, just like white dwarfs. When the materials coat the surface of the neutron star, the fuel explodes, hot enough to emit a strong burst of X rays.