The Birth and Death of Stars

By Moira Whitehouse PhD

What are Stars?

• Stars are large balls of hot gases—mainly hydrogen and helium.

• They look small because they are a long way away, but in fact many are bigger and brighter than the Sun.

• The heat of the star is made in the centre by nuclear fusion reactions.

• There are lots of different colours and sizes of star.

What are Galaxies?

• Stars form groupings called galaxies. • There are about 80-100 billion galaxies in that part of the Universe we can observe.• There may be more in the Universe beyond what we are able to detect.• Each galaxy has billions or trillions of stars.• Many astronomers believe that stars in a galaxy orbit around a black hole.

Hubble Ultra Deep Field image with a huge number of galaxies.

5 galaxies Hickson Compact Gp 52

interacting Galaxy Pair Arp 8

• Our Sun is one of the stars in the Milky Way Galaxy.

• The Milky Way Galaxy contains roughly 200 billion stars.

• The Milky Way is a spiral galaxy and the Sun in located on one of the outer arms of that galaxy.

NASA /JPL-Caltech

Artist’s image of the Milky Way

Infrared image of the Milky Way—Cosmic Background Explorer (COBE) satellite

NASA

Spiral Galaxy M7

How Are Stars “Born”?

• Stars are made (or “born”) in giant clouds of dust and gas called nebulae.

• Nebulae are located in the billions of galaxies that make up the Universe .

The Great Orion Nebula

A nebula called NGC 6302. What resembles butterfly wings are actually boiling cauldrons of gases heated to more than 36,000 degrees F.

•Sometimes part of the cloud shrinks and forms a ball as gravity pulls the gas and dust particles closer and closer together.

•The gas and dust inside the ball bump into one another causing the temperature to rise.

• Electrons are stripped off the atoms of hydrogen. The positively charged hydrogen nuclei move faster and faster crashing into one another so hard that the protons in the center or core of the nuclei fuse (join) forming helium nuclei.

• When this happens, a great deal of energy pours out of the core or middle of the ball heating the gases.

• The heated gases expand balancing the inward pull of gravity.

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• The energy moves through the outer layers of the ball and into space as electromagnetic radiation.

• The visible light causes the ball to glow

•••••••and

……… a new star is born!

Embryonic stars in the Eagle Nebula

Stars bursting into life in Carina nebula

A firestorm of stars being born in Galaxy NGC 604

Intense bursts of stars that have formed over the last few tens of million years.

• The Hodge Cluster 301 Many stars forming in the Tarantula Cluster in the Large Magellanic Cloud galaxy

What Happens During the “Life”

of a Star

• Once nuclear fusion produces heat in the centre of the new star, this heat causes

the gases to expand preventing the star from collapsing.

• During the “life” of the star the two forces continue to fight it out—outward pressure caused by nuclear fusion and gravity.

• The star stays almost exactly the same for a long time—hydrogen changing into

helium (about 10 billion years for a star like the Sun).•

• At this stage, stars are different colors, sizes and brightnesses.

• The bigger a star, the hotter and brighter it is. Hot stars are Blue. Smaller stars are less bright, cooler and Red.

• Because they are so hot, the bigger stars actually have shorter lives than the small, cool ones.

How does a Star “Die”?

• Eventually, the most of hydrogen (the “fuel” for the nuclear fusion) in the centre of the star runs out.

• Most of the hydrogen has changed to helium

• Without the heat from nuclear fusion the gases stop expanding.

• Gravity takes over and outer layers of gases collapse.

• As the star shrinks, pressure and heat in the core build up causing the gases to rapidly expand.

• For medium sized stars like our Sun they swell and become……

…………a red giant.

• As the centre collapses, the star becomes very hot again, eventually getting hot enough to start a new kind of nuclear fusion with helium as the fuel. The helium nuclei fuse now to from carbon nuclei

• Then the Red Giant shrinks and the star looks “normal” again.

• This does not last very long, though, as the helium runs out quickly and once again the star forms a Red Giant.

• The star then sheds its outer layers of gas and dust and becomes planetary nebulae.

• When a planetary nebular forms, there is only about 20% of the original star left.

Twin jet nebulaAnt nebula

Two planetary nebula

• The planetary nebula spends the rest of its life cooling and shrinking until it becomes a few thousand miles in diameter. • Finally it becomes a……

• white dwarf-- a very dense ball.

• It continues to give off heat for billions of years until it finally becomes a black dwarf and ceases to shine--you can no longer see it.

Image from the Liverpool Telescope

Here you can see a planetary nebula called M57 with its White Dwarf in the middle.

• Now we will consider what happens when a star that has more mass than the Sun “dies”.

• A more massive star follows a different sequence during this stage of its “life”.

• Even the most massive stars—those with a mass of more than 1.5 times the Sun-- run out of fuel and finally collapse.

As the core shrinks, the outer layers swell into a Red Super Giant (a very big Red Giant).

Red Supergiant Star V838 Monocerotis

Betelgeuse a red supergiant nearing its end.

Upper section of the picture is the bluish color of hot stars newly formed stars reflected by a cloud of cool gases and dust. The lower red section is Antares, a super red giant which is gradually shedding layers.

Antares – a Red Super Giant

Anatares, a Red Super Giant in the Milky Way Galaxy in the process of dying.

• Material is ejected into space where our heavier elements are formed.

• Inside the core of a Super Giant Red Star, new nuclear fusions begin causing the star to expand once more. • When the fuel for nuclear fusion finally runs out core is made of iron.

• The temperature of the core rises to 100 billion degrees Celsius and the star explodes as a supernova.

Crab Nebula with remnants of a star’s supernova explosion.

• huge stars 1.5 to 3 times the mass of the Sun shrink and form a neutron star.

After the supernova explosion:

• giant stars 3 times the mass of the Sun shrink and form a black hole.

Neutron star

This deep Chandra X-ray Observatory image shows the supernova remnant Kes. The explosion of a massive star created the supernova remnant, along with a pulsar-- a rapidly spinning neutron star.

Eta-Carinae supernova with a neutron star

A NASA image illustration of a supermassive black hole ripping apart a star and consuming part of it.

This black hole has about 3.8 times the mass of our sun, and is orbited by a companion star, as shown in this illustration. Credit: NASA/CXC/A. Hobar

The debris from a supernova explosion is blown away and forms a glowing cloud called a supernova remnant.

Remnants of a supernova

Supernova Remnant from Palomar

Remnants of a supernova and pulsar in the middle of the remnants

http://www.valdosta.edu/~cbarnbau/astro_demos/frameset_moon.html

Birth and Death of Stars - Summary

• Stars form in clouds of gas.

• Heat from nuclear fusion and gravity balance.

• When the hydrogen fuel runs out, a Red Giant or Super Red Giant forms.

• For Sun-like stars, a White Dwarf and Planetary Nebula are left.

• For massive stars, a Supernova explosion leaves behind a Supernova Remnant and a Neutron Star or perhaps even a Black Hole.