Download Death of Stars with the Mass of 0.3

Julia M. Riedl, 1211962
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Death of Stars with the Mass of 0.3-8 Sun Masses
J.M. Riedl
A star gets its energy by fusing hydrogen to helium. The fusion creates a
radiation pressure that antagonizes the gravitation which causes the star to be
stable. However, once the hydrogen in the core runs out, the star begins to fuse
helium to carbon and oxygen. Because of the high pressure, the star expands and,
if it has the right mass, becomes a red giant. If the star is light enough it collapses
after the “helium burning,” leaving a planetary nebula behind and forming a white
dwarf. If the red giant has a mass of 1.4 suns or more, it starts to fuse heavier
elements until it gets to iron. However, iron has the strongest binding energy in
the atomic nucleus of all elements; therefore it is not possible to get energy by
fusing it. The star collapses, the inner part of the core is compressed into neutrons
causing infalling material to bounce off and form an outward-propagating shock
front. While the surrounding material is blasted away in a supernova, the leftover
material forms a small star, and depending on the mass of the remaining material,
a neutron star or a black hole is formed. A supernova can also evolve from a white
dwarf that gains additional mass from the outside. When the dwarf exceeds a
critical mass, it collapses and starts a sudden fusion and explodes. In that case, no
compact object is left behind.
Being the source of all elements in the
universe that are heavier than helium i, stars
belong to the most exciting topics of
astrophysics. They are born from star forming
regions – giant nebulas – that collapse in
consequence of the gravitation. Stars live from
thermonuclear fusion until they die in an even
more spectacular way. One such worth seeing
star death – the thermonuclear supernova SN
2014J – could be seen in the beginning of this
year in the Cigar Galaxy M82. The supernova
happened about 12 million light-years away and
got spotted by astronomy students. A fainter
supernova called SN 2014L has appeared in the
spiral galaxy M99. This galaxy is four times
further away than the M99, making the
occurrence more difficult to see.ii Nevertheless,
stars not only die within a gigantic explosion
but also end silently, such as with white
dwarves. In this article I am going to explain the
main parts of the process a star of a certain mass
passes through before its death, and what it
leaves behind.
When a star, which gets its energy by burning
hydrogen to helium and has a mass of 0.3-8 sun
masses, runs out of hydrogen in the core, the
core region collapses because there is no
radiation pressure that counteracts the
gravitation. Due to this sudden compression, the
temperature rises and the fusion of hydrogen in
an outer layer begins, while the helium burning
in the core is initiated explosively. From then on
the helium fuses to carbon and oxygen. It
generates a high pressure and the outer layers of
the star expand greatly. On account of the larger
surface area, the surface temperature drops and
the star seems to be red. The star has thus
become a red giant.
When there is no helium left in the core, it
collapses again causing the temperature to rise
Julia M. Riedl, 1211962
Artikel zum Präsentationsthema
further. Therefore the helium and hydrogen in
outer layers are able to begin fusing. If the star
has enough mass, this alternating growing and
collapsing continues until it is hot enough for
heavier elements to fuse. If the red giant has
only a mass of less than about 1.4 sun masses,
the fusion stops after the helium in the core has
been burned up. Due to gravitational pressure,
the star gets compressed and becomes a white
dwarf. The outer layers are pushed away,
ionized by the UV-rays of the star and begin to
glow as a new planetary nebula. The density of
a white dwarf is extremely high – one cubic
centimeter weighs about one ton. A white dwarf
only shines because of the energy in its core.
After millions of years it will cool down until it
will slowly becomes a black dwarf.iii
If the red giant has more than 1.4 sun masses
and the helium in the inner layers is already
burned, the core collapses, provoking the
temperature to rise until the carbon can fuse to
neon and magnesium. Later, sulfur and silicium
will be formed, and the fusion of silicium
produces iron. But iron has the strongest
binding energy of the atomic nucleus of all
elements. The fusion chain stops here because it
requires more energy to fuse iron than the
process can produce. The radiation pressure
sinks and the core of the star collapses on
account of the huge gravitational pull. This
collapse frees a gigantic amount of energy –
about 1046 Joule. Only neutrons that are
produced by merging electrons and protons of
the inner core together can stop that implosion.
The rest of the material bounces off and forms a
huge explosion – a supernova. The light of a
supernova explosion is extremely high, so that it
is possible to see it as a new object in the starry
sky (stella nova = new star). The remaining
material of the star forms a pulsar – a neutron
star - or a black hole, depending on its mass.iv
If the leftover material of a supernova has
less than about 2.5 sun masses, the neutron core
Note: American English (trillion=1012)
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does not further collapse and forms a fast
spinning neutron star called a pulsar. In a
neutron star, the neutrons are packed very
tightly, causing the density of the star to be a
million times higher than the density of a white
dwarf. Neutron stars only have an average
diameter of 20 kilometers, but the weight of a
normal star.v Not only their volume but also
their magnetic field is compressed, so that a
field forces are reached that are a trillion times
higher than the magnetic field of the earth. Here
is the spinning axis of the pulsar not the same as
the axis of the magnetic field. Therefore, the
electromagnetic waves discharging from the
magnetic poles always point in another
direction. If a pole points towards the earth
during its rotation, the radio waves can be
measured frequently.vi
If the mass of the leftover material after a
supernova is higher than 2.5 sun masses, not
even neutrons can resist the strong gravitational
force, and the star is unable to regain stability.
The core collapses again until it has no volume
but infinite density and has become a black hole.
This condition is called singularity. A black hole
can be imagined as hole in space-time from
where not even electromagnetic waves can
escape. This means that black holes can only be
observed indirectly.vii
Sometimes a light star that ended up as a
white dwarf can also become a supernova. If the
white dwarf gains additional mass from outside
– for example from a companion star in a binary
star system – it can reach a critical mass and
collapse due to gravitation. The collapse leads to
a higher temperature which causes the sudden
start of carbon fusion. Through the abrupt
admission of energy, the star explodes in a
thermonuclear supernova, shooting all of the
material into the space and leaving no compact
object behind. The companion star becomes a
“runaway”-star that flies with the former orbital
velocity away.viii
i
Borgeest, Ulf: Supernovae.Sternentod mit Knalleffekt.Online im Internet:
URL: http://www.geo.de/GEO/natur/supernovae-sternentod-mit-knalleffekt-3028.html [Stand 2014-05-07].
ii
MacRobert, Alan: Supernova in M82 Passes Its Peak.Online im Internet:
URL: http://www.skyandtelescope.com/astronomy-news/observing-news/supernova-in-m82-passes-its-peak/
[Stand 2014-06-05].
iii
Rote Riesen und weiße Zwerge.Das Ende eines Sterns.Online im Internet:
URL: http://www.scinexx.de/dossier-detail-52-9.h0030062284tml [Stand 2014-05-20].
iv
Feuerwerk im Weltall.Schwere Sterne explodieren als Supernovae.Online im Internet:
URL: http://www.scinexx.de/dossier-detail-52-10.html [Stand 2014-05-20].
v
Neutronenstern.Online im Internet: URL: http://www.astronews.com/glossar/eintraege/neutronenstern.html
[Stand 2014-05-20].
vi
Superdichte Klumpen und Leuchttürme im All.Was sind Neutronensterne und Pulsare?.Online im Internet:
URL: http://www.scinexx.de/dossier-detail-52-11.html [Stand 2014-05-20].
vii
Kosmische Staubsauger.Vom Neutronenstern zum schwarzen Loch.Online im Internet:
URL: http://www.scinexx.de/dossier-detail-52-12.html [Stand 2014-05-20].
viii
Kayser, Rainer: Supernovae.Normale Sterne füttern Weiße Zwerge.Online im Internet:
URL: http://www.weltderphysik.de/gebiet/astro/news/2011/supernovae-normale-sterne-fuettern-weisse-zwerge/
[Stand 2014-05-24].