supernovae, neutron stars, and black holes

what is a supernova?

a star whose luminosity rapidly increases due to it exploding

what are the features of a supernova?

* a rapid increase in brightness
* the emission of gamma ray bursts
* an extremely high maximum brightness

explain why a supernova is not displayed on a typical HR diagram

it is too bright to be displayed on the absolute magnitude scale

why might astronomers be worried about a supernova occurring near Earth?

because supernova emit gamma-ray bursts which:

* transfer a similar amount of energy as the total energy output of the Sun in just a few seconds
* are concentrated along the star’s axis of rotation (highly collimated)

so if the axis was pointing towards Earth, the burst could cause a mass extinction event due to the ionising nature of gamma

what is a type 1a supernova?

a specific type of supernova created from a white dwarf in a binary star system increasing in mass then exploding

it has an absolute magnitude of -19.3

explain why a type 1a supernova would not appear on a typical HR diagram

it is too bright to fit on the scale

* a type 1a supernova has an absolute magnitude of -19.5
* the brightest absolute magnitude on a typical HR diagram is -10

sketch the light curve of a type 1a supernova

Define the term “standard candle” and give one example of a standard candle used in astronomy.

* an object whose absolute magnitude is known
* type 1a supernovae

how can type 1a supernovae be used as standard candles?

* if observing two galaxies that both contain type 1a supernovae
* the galaxy in which the supernova appears **brightest** is the **closest**
* as they have the same absolute magnitude

how can type 1a supernovae be used to calculate distances?

* using m-M=5log(d/10)
* M = -19.3

explain how are type 1a supernovae and dark energy related

the surprisingly high speeds at which type 1a supernovae are moving away from us provide evidence for the existence of dark energy

* measurements of type 1a supernovae found that they were travelling much faster than predicted
* so the Universe is expanding at an accelerating rate
* astrophysicists had no way of explaining this so they proposed an invisible dark energy

what are the two things that can happen after a supernova?

the star becomes either a **neutron star** or a **black hole**

state three key properties of a neutron star

* mass of around 2 solar masses
* extremely high density
* made up of neutrons (as protons and electrons combine)

what is a black hole?

an object whose escape velocity is greater than the speed of light

why can’t black holes be plotted on HR diagrams?

* too dim as no light escapes a black hole
* too low in temperature as it’s only just over 0 K

where are supermassive black holes found?

at the centre of galaxies

define Swartzchild radius

the distance from the centre of a black hole to its event horizon

what is an event horizon?

the boundary of a black hole - the point at which the escape velocity is equal to the speed of light

what is the escape velocity like **inside** the Swartzchild radius?

greater than the speed of light (hence why light cannot escape from a black hole)