Stars

King City Astronomy 1Class 4

Zoe Buck

The Most Important Things about STARS

• Stars are found in galaxies, and like to hang out in systems with other stars (binaries, open clusters, globular clusters)

• Stars are formed in giant clouds of dust and gas called molecular clouds, when denser regions of the cloud collapse due to gravity

• Before a star is hot enough to turn on nuclear fusion in its core, it is called a protostar, and is powered by gravitational collapse

• The star then enters the “main sequence,” where it burns hydrogen into helium through nuclear fusion

• When 4 hydrogen become one helium, there is some missing mass – this mass is converted into energy according to Einstein’s famous equation E=mc²

• Just like with cars, big stars run out of hydrogen fuel fast, and small stars burn through it very slowly

• Stars are made of plasma, which is not solid, not liquid, and not a regular gas – it is an ionized gas where atoms have been broken up and float around freely because it is so hot

• The Sun is a star, and we can use it to study the properties of other stars• The Sun has a core where nuclear fusion is taking place, and an atmosphere called

the Corona• The surface of the Sun that we see we call the “photosphere,” because this is

where the light we see is coming from• The surface of the Sun has granules, caused by convection in the Sun’s outer

layers, and sunspots, prominences and flares, caused by the Sun’s crazy magnetic field

The Most Important Things aboutSTARS

• Sunspots come and go over cycles of 11 years, a result of the Sun’s shifting magnetic field

• Sometimes the Sun burps giant packets of gas, charged particles and radiation called Coronal mass ejections, which can be dangerous for the Earth, but we are protected by the Earth’s magnetic field -when charged particles from the Sun hit the Earth’s magnetic field, we see Aurora

• A star’s entire life is dictated by its mass – temperature, radius, brightness, color, lifetime, and how it will die

• Very big stars, like blue giants, will burn through fuel quickly, become a red giant, and then explode in a supernova

• After a supernova, big stars will collapse due to gravity, and become neutron stars – the very biggest stars will continue to collapse because they are so massive, and will become black holes

• Medium sized sun-like stars will become red giants when they die, and then blow off their outer layers – the core of the star will remain as a white dwarf that will eventually die out

• Small stars like red dwarfs will just keep burning fuel, the Universe is not old enough for them to have died yet

• A Hertzsprung-Russel diagram organizes stars according to their brightness related to luminosoty) and spectral type (related to temperature). It can be used to tell if a star is on the main sequence, or if it is a red giant, or a white dwarf.

Solar System

Milky Way Galaxy

Local Group of galaxies

Virgo Supercluster

Galaxies are made up of Gas, Dust, Dark Matter, and STARS

Stars like to be together

Open Cluster Globular Cluster

Binary Systems

binary stars: two stars bound by gravity and orbiting each other around their center of gravity (double star)

Where do stars come from?

Star formation – Molecular Clouds

• There is a lot of gas in space• Most of that gas is Hydrogen (H)• There are clouds in space that are dense

enough that Hydrogen atoms get together to form molecules

• 2 H atoms create 1 H2

• These “molecular clouds” are where star formation takes place

• Molecular clouds are one kind of “nebula”

Star Formation – Molecular Clouds

Star Formation

• Stars are born in giant molecular clouds

• Some regions in the cloud are denser than other regions

• Because of the high density, these regions collapse due to gravity

Star Formation - Protostars

• Protostars are baby stars• After it has fragmented from a gas

cloud

Protostars

A Star is born when the protostar “turns on” and begins to produce its own energy

Is the Sun on fire?

• NO, there is no combustion of fuel, no air, no smoke

Star Music Videos

• The Sun is a Mass of Incandescent Gas

• The Sun is a Miasma of Incandescent Plasma

Plasma?

http://www.youtube.com/watch?v=qvSxVBalhFM

The Surface of the Sun

• We call the surface of the Sun the photosphere

• Granules – the top of convective cells

Convection

The solar atmosphere – The Corona

Magnetism

• Magnetism is the key to understanding the Sun

• Magnetic fields are produced in the Sun by the flow of electrically charged ions and electrons

Sunspots and Solar Activity

• Sunspots are places where very intense magnetic lines of force break through the Sun’s surface

Sunspots and solar activity

• Solar activity goes through cycles• The sunspot cycle results from the

recycling of magnetic fields by the flow of material in the interior

Sunspots and solar activity

Sunspots and Solar Activity

• The prominences seen floating above the surface of the Sun are supported, and threaded through, with magnetic fields

• The streamers and loops seen in the corona are shaped by magnetic fields

• Flares are explosions of gas and radiation on the surface of the Sun

Coronal Mass Ejection (CME)

• Coronal Mass Ejections are enormous bubbles of gas, give off ultraviolet light and x-rays that can be dangerous

The Sun’s magnetic field sends dangerous particles our way; the Earth’s magnetic field protects us

We see the effects of this as aurora

Energy production – Nuclear Fusion

• Stars are fueled by the energy released in nuclear fusion

• Nuclear fusion occurs when smaller atoms fuse together to make larger atoms

• In the center of stars, hydrogen (H) fuses together to create helium (He)

• The center of an atom is called a “nucleus”

• 4 H nuclei fuse to become 1 He nucleus, releasing energy

Where does the energy come from? Why does 4 H nuclei

fusing into He release energy?

E=mc²

• The missing mass is converted into energy• This is what powers the Sun and other

stars, mass being converted into energy during nuclear fusion

• This was one of Einstein’s most famous contributions

Star Fuel

• Hydrogen is star fuel• Helium is star byproduct

Star Fuel

Spectral Types of Stars

O B A F G K M

Spectral Types of Stars

A Pattern between brightness and temperature

• Hertzsprung and Russell noticed a pattern between spectral type and brightness of a star

The Main Sequence

• Stars on the Main Sequence are not babies, nor are they old – they are in the prime of their lives

• Stars on the main sequence have achieved a balance between gravity, trying to make them collapse in on themselves, and nuclear fusion

• This balance is called “hydrostatic equilibrium”

The Main Sequence

• Only stars on the main sequence are fueled by stable nuclear fusion, burning hydrogen into helium to maintain hydrostatic equilibrium

Mass and evolution

• All stars begin as Protostars and then move onto the Main Sequence.

• Where on the main sequence a star ends up is determined by its MASS.

• MASS determines a star’s color, spectral type (remember these are closely related) temperature and brightness.

• It also determines what happens later, when the star runs out of fuel, leaves the Main Sequence and begins to die.

Small Medium LargeRed Dwarf Sun-like star Blue Giant

A star enters Old Age - Red Giant

Betelgeuse

Red Supergiant

Stellar remains

• What is left after a star dies (the stellar remains) depends on how the star died

• How the star died depends primarily on its mass during life

Stellar Remains – Planetary Nebula

Supernovae

What’s left after a supernova?

Neutron Stars

• When a star has enough mass, and there is no fusion to keep it in balance (hydrostatic equilibrium), the star collapses in on itself

• Neutron stars are incredibly dense: the equivalent of the entire human population compressed to the size of a sugar cube

• Neutron stars spin very fast because they have been so compacted, like an ice skater who pulls in his or her arms

Density of a Neutron Star

Black Holes

• When the mass of a stellar remnant is between 10 and 25 times the mass of the Sun it will become a black hole, a region so dense that not even light can escape its gravity if it gets close.

Gravitational lensing

What about the small stars, the red dwarfs?

The Most Important Things about STARS

• Stars are found in galaxies, and like to hang out in systems with other stars (binaries, open clusters, globular clusters)

• Stars are formed in giant clouds of dust and gas called molecular clouds, when denser regions of the cloud collapse due to gravity

• Before a star is hot enough to turn on nuclear fusion in its core, it is called a protostar, and is powered by gravitational collapse

• The star then enters the “main sequence,” where it burns hydrogen into helium through nuclear fusion

• When 4 hydrogen become one helium, there is some missing mass – this mass is converted into energy according to Einstein’s famous equation E=mc²

• Just like with cars, big stars run out of hydrogen fuel fast, and small stars burn through it very slowly

• Stars are made of plasma, which is not solid, not liquid, and not a regular gas – it is an ionized gas where atoms have been broken up and float around freely because it is so hot

• The Sun is a star, and we can use it to study the properties of other stars• The Sun has a core where nuclear fusion is taking place, and an atmosphere called

the Corona• The surface of the Sun that we see we call the “photosphere,” because this is

where the light we see is coming from• The surface of the Sun has granules, caused by convection in the Sun’s outer

layers, and sunspots, prominences and flares, caused by the Sun’s crazy magnetic field

The Most Important Things aboutSTARS

• Sunspots come and go over cycles of 11 years, a result of the Sun’s shifting magnetic field

• Sometimes the Sun burps giant packets of gas, charged particles and radiation called Coronal mass ejections, which can be dangerous for the Earth, but we are protected by the Earth’s magnetic field -when charged particles from the Sun hit the Earth’s magnetic field, we see Aurora

• A star’s entire life is dictated by its mass – temperature, radius, brightness, color, lifetime, and how it will die

• Very big stars, like blue giants, will burn through fuel quickly, become a red giant, and then explode in a supernova

• After a supernova, big stars will collapse due to gravity, and become neutron stars – the very biggest stars will continue to collapse because they are so massive, and will become black holes

• Medium sized sun-like stars will become red giants when they die, and then blow off their outer layers – the core of the star will remain as a white dwarf that will eventually die out

• Small stars like red dwarfs will just keep burning fuel, the Universe is not old enough for them to have died yet

• A Hertzsprung-Russel diagram organizes stars according to their brightness related to luminosoty) and spectral type (related to temperature). It can be used to tell if a star is on the main sequence, or if it is a red giant, or a white dwarf.

More videos

• Carl Sagan on the life of a star: http://www.youtube.com/watch?v=XRMINy-tUp8&feature=endscreen&NR=1