Not to be confused with
Nova
.
A
supernova
is the explosion of a
giant star
. It usually happens when the
nuclear fusion
in the star cannot hold the core against its own gravity.
[1]
The core collapses, and then explodes.
The biggest stars that make supernovae are
hypergiants
and smaller ones are
supergiants
. They are
massive
: because of
gravity
, they use up their energy very quickly. Normally they only live for a few million years.
During the explosion, the total energy radiated by supernovae may briefly outshine the entire output of a
galaxy
. They
emit
energy equal to that of the whole lifetime of a solar-like star.
[2]
The explosion blows off its stellar material away from the star, at velocities up to 30,000 km/s or 10% of the speed of light.
[3]
This drives a
shock wave
into the surrounding
interstellar medium
. This sweeps up an expanding shell of gas and dust, which we see as a supernova remnant. After exploding, what is left becomes a
black hole
or a
neutron star
.
Most stars are small and do not explode. After their
red giant
phase, they become colder and smaller and become
white dwarf
stars.
[4]
Supernova explosions happen rarely. The last time people saw a supernova in our own
galaxy
, the
Milky Way
, was in the year
1604
. We can see supernovas in other galaxies too. Every year we see 300 supernovas in other galaxies, because there are so many galaxies. Sometimes they are brighter than the whole rest of the galaxy.
Supernovas are usually sorted into
Type I
and
Type II
supernovas.
[5]
Type I
supernovas have
absorption lines
that show they do not have
hydrogen
in them.
Type Ia
supernovas are very bright for a short amount of time. Then they get less bright very quickly. Type Ia supernovas happen when a
white dwarf
star is
orbiting
a big star. Sometimes, the white dwarf star sucks
matter
off of the big star. When the white dwarf eventually gets to be about 1.4 times the
mass
of the sun, it
collapses
. This makes lots of energy and light, which is why supernovas are very bright. Type 1a have mostly the same brightness. This allows them to be used as a secondary
standard candle
to measure the distance to their host galaxies.
[6]
Type II
supernovas have absorption lines that show they do have hydrogen in them. A star must have at least 8 times, and no more than 40?50 times, the
mass of the Sun
to undergo this type of explosion.
[7]
In a star like the Sun,
nuclear fusion
turns
hydrogen
into
helium
. In very large stars,
helium
gets turned into
carbon
, then
oxygen
, and so on. The star fuses increasingly higher mass elements, up through the periodic table until a core of
iron
and
nickel
is produced. Fusion of iron or nickel produces no net energy output, so no more fusion can take place. When the energy released from the fusion process fades it cannot counteract gravitational collapse. The collapse of the core is so rapid (about 23% of the speed of light) that a huge shock wave is produced. The extremely high temperature and pressure lasts long enough for a brief moment when the elements heavier than iron are produced.
[8]
Depending on initial size of the star, the remnants of the core form a
neutron star
or a
black hole
.
Without supernovas there would be no life on Earth. This is because many of the
chemical elements
were made in supernova explosions. These are called "heavy elements". Heavy elements are needed to make living things. The supernova is the only natural way
heavy elements can be made
. Other elements were made by
fusion
in stars. Heavy elements need very high
temperature
and
pressure
to form. In a macho supernova explosion the temperature and pressure are so high that heavy elements can be made. Scientists call this
supernova nucleosynthesis
.
It could be dangerous if a supernova explosion happened very close to the Earth. The explosion is very big and many kinds of dangerous
radiation
are formed. But we do not have to be afraid. Only very big stars can explode as supernovas. There are no stars big enough near the Earth and if there was it would take millions of years for it to happen.
SN 1572
was seen by
Tycho Brahe
. This supernova helped astronomers learn that things in space could change.
[9]
SN 1604
was seen by
Johannes Kepler
. It was the last supernova close enough to be seen from Earth's
northern hemisphere
without a
telescope
.
[10]
SN 1987A
is the only supernova so close that scientists could find
neutrinos
from it.
[11]
SN 1987A
was also bright enough to see without a telescope. People in the
southern hemisphere
saw it.
The Earth does have traces of past supernovae. Traces of
radioactive
iron-60, a strong indicator of supernova
debris
, is buried in the sea floor right across the globe.
The "local bubble" is a ballooning region of hot gas, 600 light-years across. It surrounds the
Solar System
and dominates our stellar neighbourhood. It was formed by over a dozen supernovae blowing up in a nearby moving clump of stars. This happened between 2.3 million and 1.5 million years ago.
[12]
This roughly corresponds with the start of the
Pleistocene
ice ages
. The connection may be accidental.
- ↑
This is the cause of all types of supernova except type Ia.
- ↑
Giacobbe, Frederick W. 2005. How a Type II supernova explodes.
Electronic Journal of Theoretical Physics
2
(6): 30?38.
[1]
- ↑
Heger, Alexander
et al
2003. How massive single stars end their life.
Astrophysical Journal
.
591
: 288.
[2]
- ↑
Tyler, Pat (26 June 2003).
"Supernova"
. NASA HEASARC: Education and Public Information
. Retrieved
29 January
2013
.
- ↑
"Supernovae"
. Georgia State University
. Retrieved
29 January
2013
.
- ↑
Colgate, Stirling A. (1979). "Supernovae as a standard candle for cosmology".
The Astrophysical Journal
.
232
(1): 404?408.
Bibcode
:
1979ApJ...232..404C
.
doi
:
10.1086/157300
.
- ↑
Gilmore, Gerry (2004). "The short spectacular life of a superstar".
Science
.
304
(5697): 1915?1916.
doi
:
10.1126/science.1100370
.
PMID
15218132
.
S2CID
116987470
.
- ↑
Staff (2006-09-07).
"Introduction to Supernova Remnants"
. NASA Goddard/SAO
. Retrieved
2007-05-01
.
- ↑
Calar Alto Observatory-CAHA (4 December 2008).
"Blast from the past: astronomers resurrect 16th-century supernova"
. Science Daily
. Retrieved
29 January
2013
.
- ↑
Frommert, Hartmut; Kronberg, Christine.
"SN 1604, Kepler's Supernova"
. Retrieved
29 January
2013
.
- ↑
"Twenty Years after SN1987a"
. 25 February 2007
. Retrieved
29 January
2013
.
- ↑
Webb, Jonathan 2015. Exploding stars left recent, radioactive mark on Earth.
BBC News
Science & Environment.
[3]