Supernova Remnant Profile
The discussion on this page relates to stars with masses of more
than 5 times that of the Sun. For more information on life-cycles of
all types of stars, visit the Imagine the Universe! page on stars:
Stars with masses of at least 5 times that of our Sun end their lives
in a massive explosion, called a supernova (plural is supernovae).
These bright, violent explosions are the key to understanding how
heavier elements spread across the Universe.
Think of the elements that make up the world around us - carbon makes up
organic material, gold in jewelry, oxygen in the air we breathe. What
other examples can you think of?
In the Big Bang, hydrogen and helium were the primary elements made
(about 75% by mass hydrogen, 25% helium, 0.01% deuterium and trace (on
the order of 10-10) amounts of lithium and beryllium). As you noticed
above, very little of what we see around us is made up of hydrogen and
helium. Somehow, hydrogen and helium must have transformed into the
The key is fusion. Fusion is the energy source at the centers of
From dust to dust the life-cycle of a massive star
Hubble image of a section of the Eagle Nebula. The Eagle Nebula is a
giant stellar nursery.
For more information on this image visit the original Hubble news release (http://hubblesite.org/newscenter/archive/releases/nebula/2005/12/image/b/)
A star starts its life when a cloud of dust and gas collapses to form
a proto-star. As a proto-star gains mass, the gravity pulls matter
toward the core of the proto-star. The pressure at the center of the
proto-star will increase, as will its temperature, the more mass it
gains. Once the temperature reaches about 15 million degrees Celsius,
fusion of hydrogen into helium can begin, and the proto-star becomes a
During most of its live, the star will fuse hydrogen into helium.
Gravity continues to pull material toward the center of the star, but
the energy released in fusion will keep the star from collapsing. The
primary fusion reaction is:
4(1H) ⇒ 4He + 2 e+ + 2 neutrinos + energy
The extra energy released in the reaction appears because the mass of
the four 1H atoms is less than the mass of one 4He
atom. The extra mass turns into energy using Einstein's famous
After a few tens of millions of years, most of the
hydrogen will be fused into helium in the core of the star. Since
fusion holds the star up against gravitational collapse, the core will
collapse in the absence of hydrogen fusion. The kinetic energy of that
collapse turns into heat, which causes the outer layers of the star to
expand. The star turns into a red giant.
However, as the core collapses, the temperature and pressure of the
center of the star will increase. When it reaches about 100 million
degrees Celsius, the helium in the core will start to fuse into carbon.
Balance is restored, so the star does not collapse completely. Instead,
it will spend some time fusing helium into carbon.
Hubble image of supernova remnant N 49 in the Large Magellanic Cloud.
Credit: NASA and The Hubble Heritage Team (STScI/AURA)
For more information on this image visit the original Hubble news release (http://hubblesite.org/newscenter/archive/releases/nebula/supernova%20remnant/2003/20/image/a/)
After some time, the helium in the star's core will be used up,
causing the core to collapse again. If the star is massive enough, the
core will get hot enough for the carbon to fuse into oxygen, and the
process repeats itself. In fact, stars that have enough mass will
successively fuse hydrogen to helium, helium to carbon, carbon to
oxygen, oxygen to neon, neon to magnesium, magnesium to silicon, and
silicon to iron.
The fusion process in stars stops at iron. There are lots of
elements on the periodic table heavier than iron, so why does the fusion
process stop at iron? Up to this point, the fusion reactions release
energy. However, fusing iron atoms requires an input of energy.
So, once the core of the star has been fused into iron, it can no longer
gain energy through fusion to battle gravity.
With the battle against gravity lost, the star collapses, and in the
processes it explodes as a supernova.
If the star died quietly, just cooling off like a used piece of coal,
those newly-formed elements would be forever locked up inside the star.
The supernova explosion is key to freeing the newly-formed elements from
inside the star, dispersing them into the star's surroundings. In
addition, the explosion itself can cause other elements to form,
elements heavier than iron.
Supernovae leave behind remnants of hot gas that astronomers can
study. The image below is a picture of a supernova remnant in visible
Image of a supernova remnant known as Cassiopeia A, or Cas A for
short. This image was taken by the Hubble Space Telescope, and shows
the remnant in visible light. For more information on this image,
please visit the original Hubble news release (http://hubblesite.org/newscenter/archive/releases/nebula/supernova-remnant/2006/30/)
The gas in a supernova explosion is hot, so they emit X-rays in
addition to visible light. By studying the X-ray light, astronomers can
see different things when looking in the X-ray. Below is an image of
the same supernova remnant in the X-ray.
Image of Cas A taken by the Chandra X-ray Observatory, and shows the
remnant in visible light. For more information on this image, please
visit this link: Cassiopeia A:Elemental Image Of Exploded Star (http://chandra.harvard.edu/photo/2000/cas_a062700/)
Notice that different areas of the image show up brighter in X-rays
than they did in visible light.
Studying Supernovae Elements in the Universe
By studying supernovae, astronomers are studying the locations where
elements have been formed in the Universe. In the rest of this activity
you will study the spectrum of a supernova in the X-ray to see what
elements are present. First, check out the "What is Spectroscopy" link below
to find out more about spectra and what you can learn from them.