Star Life Cycle

Hello everyone!

In the past couple of posts, we have talked about some of the beginning and endings of a stars life. In this post we will talk about its life cycle.

Figure 1: A simple version of a stars life cycle. Click to enlarge.

A star’s life cycle is determined by its mass. The larger the mass, the shorter the life cycle. Figure 1 shows an extremely simplified version of this cycle. A star’s mass is determined by the amount of matter that is available in its nebula, the giant cloud of gas and dust in which it is born. Over time, gravity pulls the hydrogen gas in the nebula together and it begins to spin. As the gas spins faster and faster, it heats up and is known as a protostar. Eventually the temperature reaches 15,000,000 °C and nuclear fusion occurs in the cloud’s core. The cloud begins to glow brightly. At this stage, it contracts a little and becomes stable. It is now called a main sequence star and will remain in this stage, shining for millions or billions of years to come.

As the main sequence star glows, hydrogen in the core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take after the red giant phase.

MEDIUM STARS

Throughout the red giant phase, the hydrogen gas in the outer shell continues to burn and the temperature in the core continues to increase. At 200,000,000 °C the helium atoms in the core fuse to form carbon atoms. The last of the hydrogen gas in the outer shell is blown away to form a ring around the core. This ring is called a planetary nebula. When the last of the helium atoms in the core are fused into carbon atoms, the medium size star begins to die. Gravity causes the last of the star’s matter to collapse inward and compact. This is the white dwarf stage. At this stage, the star’s matter is extremely dense. White dwarfs shine with a white hot light. Once all of their energy is gone, they no longer emit light. The star has now reached the black dwarf phase in which it will forever remain.

MASSIVE STARS

Once massive stars reach the red giant phase, the core temperature increases as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull carbon atoms together as the temperature increases forming oxygen, nitrogen, and eventually iron. At this point, fusion stops and the iron atoms start to absorb energy. This energy is eventually released in a powerful explosion called a supernova. A supernova can light up the sky for weeks. The temperature in a supernova can reach 1,000,000,000 °C. The core of a massive star that is.


1.5 to 4 times as massive as our Sun ends up as a neutron star after the supernova. Neutron stars spin rapidly giving off radio waves. If the radio waves are emitted in pulses (due to the star’s spin), these neutron stars are called pulsars. The core of a massive star that has 8 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core, so it is swallowed by its own gravity. It has now become a black hole which readily attracts any matter and energy that comes near it.

Answers to previous questions:

1. (Your own answers)
2. Latin for cloud
3. 1054 AD (by Chinese astronomers)
4. Dark Nebula
5. When planetary nebulae are formed, how fast do they expand?

Research/Review Questions:

1. Do black dwarves actually exist?
2. Order hottest to coldest, the colour of stars.
3. What is nuclear fusion?
4. What is the closest nebula to earth?
5. What nebula is the sun theorised to have come from?

Q: Why couldn't the astronaut see?
A: He had a twinkle in his eye.

Enjoy, :)

Emily.

Nebulae

Hello everyone!

In the last 2 posts, we have talked about Black Holes, at the end of a stars life. In this post, we will talk about the beginning of a stars life, Nebulae.

Figure 1: Horsehead Nebula

Just where these hot balls of gas start their lives begins in what astronomers call a nebula (plural: nebulae) and they are basically the nurseries of the Universe.  Nebulae are gigantic cloud of dust and gas; mainly of hydrogen and helium gases, and they can be light years across. They look quite fuzzy in appearance – pretty much like fluffy clouds or cotton wool in the sky. Nebulae come in not just a variety of sizes, they also come in a range of shapes with some of them looking very much like anything from horses (figure 1) to eyes (figure 2). The massive question is though, how do they form or have they always been there?

Figure 2: Cats Eye Nebula

When it comes to making stars, astronomers believe that nebulae are made from the huge collapse of gas in what they refer to as the Interstellar Medium. As the material falls in on itself under its own weight, large stars are made in the centre. When this happens, ultraviolet radiation shoots out like a laser beam and the nebula is lit up – just like a Christmas tree! This is an Emission Nebula.

There are many famous emission nebulae, one of them is probably one that you have heard of and is easily one of the most well known; astronomers call this the Orion Nebula (figure 3) because you can find it in the constellation of Orion. This type of nebula is very, very hot because of the hot, newborn stars that zap their surroundings with sizzling rays of hot particles with lots of energy – much like how the Sun throws out hot beams on our planet but only so much hotter! Emission nebulae are usually found to glow red or pink in colour – this is because they are filled with lots and lots of hydrogen gas.

 

Figure 3: Orion Nebula taken by the Hubble and Spitzer telescopes

Sometimes, stars do not have enough pent-up energy to zap their surroundings with high energy particles. So they happily sit in clouds of dust and these clouds reflect their light. You might be able to see little pieces of dust floating around in the air right now. Do you know why you can see them? Because there’s light shinning on them. This light is reflected and that’s pretty much what happens in a reflection nebula. Where there’s a reflection nebula (figure 4), you can usually guarantee that an emission nebula is not too far away. Astronomers call them diffuse nebulae when they are found together. Light that comes from the Sun and most newborn stars is called white light and it is made of many different colours – very similar to a rainbow. When this light travels passed particles of dust, the blue light (or if you want to be scientific; wavelength) is scattered; bouncing off every dust particle that it encounters before reaching our eyes – very much like how balls on a pool table are thrown in all directions as the white ball hits them and they start to bounce off of each other if they come into contact. That’s why we see blue in the sky above us or in reflection nebulae.

Figure 4: Reflection Nebula NGC

Last but not least there’s planetary nebulae – but do not be fooled – these shells of gas have absolutely nothing to do with planets! This type of nebula earns its name because some astronomers of the 18^th Century believed that they looked like giant worlds through the eyepiece of small telescopes. Here’s a tricky question; how do you think that these nebulae are made? Here’s a clue – it’s not through the collapse of the interstellar medium and it’s something to do with the fuel of a star.

Planetary nebulae (figure 5) are made when a star runs out of fuel to burn. What happens next is amazing. It’s not quite the same as when your car runs out of fuel, where it stops moving – what happens to a star is quite a bit different; it blows off its outer layers of gas in the shape of a ring or bubble. When stars do this, astronomers say that a star is dying. But it is not a sad ending for a star, it’s a beautiful colourful one! Planetary nebula are usually visible for around 50,000 years before starting to mix in with the space that surrounds it – so there is plenty of time to get out your telescope to have a look.

Figure 5: Hourglass Nebula

Answers to previous research questions:

1. William Herschel in 1800
2. He was studying the temperature of different colours by moving a thermometer through a split prism. He noticed that the highest temperature was beyond red. He theorised that this temperature change was due to "Calorific rays" which would be in effect a type of light ray that could not be seen.
3. Infrared
4. 3723 (3/11/13)
5. Fermi Gamma-Ray Space Telescope and Swift Gamma Ray Burst Explorer

Research Questions:

1. What are some other famous Emission Nebulae?
2. Where does the word "Nebula" come from?
3. What year was the Crab Nebula formed?
4. What are the dense areas of gas and dust that block out any light coming from behind called?
5. When planetary nebulae are formed, how fast do they expand?

Q: What do you call a looney spaceman?

A: An astronut!

Enjoy :)

Emily.