Imagine the Universe!

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.

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 heavier elements.

The key is fusion. Fusion is the energy source at the centers of stars.

From dust to dust – the life-cycle of a massive star

Hubble image of the Eagle Nebula

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

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 full-fledged star.

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 equation: E=mc2.

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

Hubble image of supernova remnant N 49 in the Large Magellanic Cloud.


For more information on this image visit the original Hubble news release

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.

Dispersing elements

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 light.

Image of the Cas A supernova remnant taken by Hubble

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

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 the Cas A supernova remnant taken by Chandra

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

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.



The Imagine Team
Project Leader: Dr. Barbara Mattson
Curator: Meredith Gibb
Responsible NASA Official: Phil Newman
This page last updated: Thursday, 25-Mar-2010 08:54:59 EDT