• Stars differ in:
• Brightness
• Colour
• Mass

• What factors influence a star’s brightness?

• What factors influence a star’s brightness?
1. Star’s Distance from Earth
2. Size
3. Luminosity
( The closest stars to Earth are not necessarily the brightest )
Luminosity – a measure of the total amount of energy a star radiates per second.

• Not all stars are the same colour!
• Pale blues, greens, yellows, orange-reds
What can colour indicate about a star?

• What can colour indicate about a star?
1. Temperature
2. Chemical Composition (what the star is made of)
( Chemical composition determined by spectroscopy )

• There are many forms of electromagnetic radiation
(from short wavelength gamma rays to long wavelength radio waves).
• The human eye can detect visible light
(wavelengths corresponding to the colours
ROYGBV).

Visible colour and Temperature
Red Yellow Blue
(In order of increasing temperature)
• Red = long wavelength = low energy
• Yellow = medium energy
• Blue = short wavelength = high energy
As wavelength increases energy decreases and with that temperature

The Relationship between
Temperature and Colour
Astronomers use the colour of a star to infer its surface temperature.
Colour Temperature (K)
Blue
Blue-white
Above 31000
9750 – 31000
White 7100 – 9750
Yellow-white 5950 – 7100
Yellow 5250 – 5950
Orange
Red
3800 – 5250
2200 – 3800

• Light from a star or galaxy can be divided into its component wavelengths (colours) using a spectroscope
• Spectroscopy is the analysis of light by breaking it down into its component wavelengths (bands of different colours)

Each element emits or absorbs some wavelengths of light leaving a unique pattern of bands in the spectrum (like a fingerprint or a bar code)
From these spectral patterns scientists can determine the composition of a star

• Use a small spectroscope slide to view the line spectrum of white light and then several electrified gases.

Our theory must explain the data
1. Large bodies in the Solar System revolve and rotate in the same direction.
2. There are two types of planets.
– small, rocky terrestrial planets
– large, hydrogen-rich Jovian planets
3. Asteroids & comets exist in certain regions of the Solar System
4. There are exceptions to these patterns.

• The nebular theory holds that our Solar System formed out of a nebula which collapsed under its own gravity.
Solar Nebula - A cloud of gas and dust
• observational evidence
– We observe stars in the process of forming today.
– They are always found within interstellar clouds of gas.
newly born stars in the Orion
Nebula

1. The solar nebula was initially somewhat spherical and a few light years in diameter.
– very cold
– rotating slightly
2. It was given a “push” by some event.
– perhaps the shock wave from a nearby supernova
3. As the nebula shrank, gravity increased, causing collapse.
4. As the nebula “falls” inward, gravitational energy is converted to heat.
5. As the nebula’s radius decreases, it rotates faster

• As the nebula collapses, it heats up, spins faster and flattens.
• The spinning nebula assumes the shape of a disk.
• As the nebula collapses, clumps of gas collide & merge.

• The Sun formed in the very center of the nebula.
– temperature & density were high enough for nuclear fusion reactions to begin
• The planets formed in the rest of the disk.
• This would explain the following:
– all planets lie along one plane (in the disk)
– all planets orbit in one direction (the spin direction of the disk)
– the Sun rotates in the same direction
– the planets would tend to rotate in this same direction
– most moons orbit in this direction
– most planetary orbits are near circular

• We have observed disks around other stars.
• These could be new planetary systems in formation.

• Condensation – elements & compounds began to condense
(i.e. solidify) out of the nebula….
depending on temperature!

So only rocks & metals condensed within 3.5 AU of the
Sun… the so-called frost line .
Hydrogen compounds (ices) condensed beyond the frost line.

Accretion -- small grains stick to one another via electromagnetic force (imagine “static electricity”) until they are massive enough to attract via gravity to form planetesimals.

Planetesimals then will:
• combine near the Sun to form rocky planets
• combine beyond the frostline to form icy planetesimals which…
• gravitationally capture H/He far from Sun to form gas planets

Building the Planets V: Jovian planets and their moons
• Each gas (Jovian) planet formed its own “miniature” solar nebula.
• Moons formed out of the disk.

(summary)
• All stars begin as a nebula, a large cloud of gas (mostly hydrogen) and dust.
• This cloud collapses inward under its own gravity.
• The heat and compression leads to a protostar and eventually hydrogen fusion ignition.

• Complete your worksheet detailing the “birth” of a star.

• A star is “born” when hydrogen fusion begins.
• Fusion creates a huge outward, expanding pressure. This is countered by the inward force of gravity. Thus, a star remains a constant size on the main sequence of a H-R diagram.

Hertzsprung-Russell Diagram
• A graph of star properties that charts luminosity against colour
(temperature).
• Working separately, Russell and Hertzsprung found that when they plotted luminosity against colour, the stars fell into distinct groups.

How to interpret the diagram.
• A star enters the diagram somewhere in the main sequence and then moves off the sequence when it runs out of fuel.
• How long it stays on the main sequence and where it moves to depends on size.

• Depends on initial size.
• Is it a…
• Low mass star
• Medium mass star, or
• High mass star

• Fuses hydrogen into helium for hundreds of billions of years.
• After running out of fuel, these stars contract due to gravity and heat up becoming white dwarfs.
• They will eventually cool to black dwarfs.

• Mid-mass stars spend their mainsequence lives fusing hydrogen into helium in their cores. (50 billion years)
• When the core runs out of hydrogen, the push outward due to fusion decreases and gravity contracts the star causing fusion to begin in a shell of hydrogen surrounding the core.
• Shell-hydrogen burning takes place at a higher rate than hydrogen fusion did during the stars main-sequence life.
• As shell-hydrogen burning proceeds, the core and the burning shell of hydrogen continue to contract, while the outer layers of the star expand producing a red giant .

Red giant to white dwarf
• When the core of a low-mass star reaches 100 million K, helium fusion begins in the core.
• The burning helium core pushes the shell of burning hydrogen outward, lowering its temperature and its burning rate, and the star contracts.
• Shell helium burning later starts and the star expands again.
• Eventually, the star sluffs off its outer layers forming a planetary nebula , and the star contracts to become a white dwarf .

• Hot, bright, blue stars live a relatively short life (30 million years)
• Cores complete many fusion reactions:
• Hydrogen →helium → carbon → neon → silicon → iron
• when fusion stops, gravity takes over and the star collapses, recoiling into a supernova explosion.

• Only occurs in large stars when they use up their fuel.
• The star first collapses on itself
(due to gravity), and then explodes outward with great force.
• During this time, it shines so brightly that it can be seen during the day.
• A supernova star will either become a neutron star or black hole.

• Occurs when the star is 5X bigger than our Sun.
• A small but extremely dense core spinning fast.
• The spinning generates a magnetic field and the star spews out radiation like a lighthouse beacon.

• Occurs when the star is 10X bigger than our
Sun.
• Created when a massive star collapses due to gravity into a single point.
• At this point, called the singularity, pressure and density are infinite.
• When anything gets too close to the event horizon, it gets pulled in and cannot escape.

• Complete the worksheet on the lifespan of stars.