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Home , 12 Cancri, Blue giant, Chromosphere, Photosphere

Exam #2 Review

Looks “quiet in the visible” • The is a huge ball of The Sun gas at the center of the – 100 times diameter, 300,000 earth mass – 1 million would fit inside it! – Releases the equivalent of 100 billion atomic bombs every second! – 1366 watts/square meter at Earth – 15 Tera watts in 62 sq mi • Exists thanks to a delicate balance of and pressure

1 Knowledge of interior based on models which fit observables: •Mass •Radius • •Surface •Image details: granules, spicules, corona,

The

• The photosphere is the visible “surface” of our – Not really a surface, as the Sun is gaseous throughout – Photosphere is only 500 km thick – Average temperature is 5780 K

2 Energy Transport in the Sun

• Just below the photosphere is the zone. – Energy is transported from deeper in the Sun by convection, in patterns similar to those found in a pot of boiling water (hot gas rises, dumps its energy into the photosphere, and then sinks) • Energy in the comes from the radiative zone. – Energy from the core is transported outward by radiation – Takes more than 100,000 for a single photon to escape the Sun!

The Solar

• Regions of the Sun above the photosphere are called the Sun’s atmosphere • Just above the photosphere lies the chromosphere – Usually invisible, but can be seen during • Above the chromosphere is the corona – Extremely high (more than 1 million K!) – Rapidly expanding gas forms the .

3 The Ideal Gas Law

Pressure = Constant × Temperature ×

The Sun’s Energy • The Sun’s energy comes from fusion – the merging of nuclei into • The reaction releases only a little bit of energy, but it happens a lot! • A hydrogen nucleus has less mass than the four protons (hydrogen nuclei) that fuse

• This difference in mass is converted into energy:

E = m×c2 The Proton-Proton Chain

4 • Sunspots are highly localized cool regions in the photosphere of the Sun – Discovered by Galileo – Can be many times larger than the Earth! – They contain intense magnetic fields.

Solar Flares

5 • When CME material reaches the Earth, it The Aurora gets funneled by Earth’s and collides with ionospheric particles, close to the poles • The collision excites ionospheric oxygen, nitrogenand which causes it to emit a photon • We see these emitted photons as the aurora, or Northern

6 Measuring the Distance to Astronomical Objects using parallax

Just a little

Trigonometry1 = …206265 AU

7 Moving

• The positions of stars are not fixed relative to Earth – They move around the center of the , just as Earth does. – This motion of stars through the sky (independent of the Earth’s rotation or orbit) is called – Over time, the will change shape! • The speed of a star’s motion toward or away from the Sun is called its

The Inverse-Square Law

• A star emits in all directions, • The brightness decreases with like a light bulb. We see the the square of the distance from photons that are heading in our the star direction – If you move twice as far from • As you move away from the star, the star, the brightness goes fewer and fewer photons are down by a factor of 22, or 4! heading directly for us, so the star seems to dim – its brightness • Luminosity stays the same – the decreases. total number of photons leaving a sphere surrounding the star is constant.

8 You see this every day!

• More distant streetlights appear dimmer than ones closer to us. • It works the same with stars! • If we know the total energy output of a star (luminosity), and we can count the number of photons we receive from that star (brightness), we can calculate its distance L d = • Some types of stars have a4 !knownB luminosity, and we can use this standard candle to calculate the distance to the neighborhoods these stars live in.

Measuring Temperature using Wein’s Law

2.9#106 K "nm T = !

9 The Stefan-Boltzmann Law flux = "T 4

Flux is energy / unit area Where, σ= 5.67×10−8 W·m-2·K-4

L = flux • Area = "T 4 • 4# r2

• The Stefan-Boltzmann! Law links • A star’s luminosity is then a star’s temperature to the related to both a star’s size and amount of light the star emits a star’s temperature – Hotter stars emit !more! • We need an organizational tool – Larger stars emit more! to keep all of this straight…

Photons in Stellar

• Photons have a difficult time moving through a star’s atmosphere • If the photon has the right energy, it will be absorbed by an atom and raise an to a higher energy level • Creates absorption spectra, a unique “fingerprint” for the star’s composition. The strength of this spectra is determined by the star’s temperature.

10 Classify:

1- variation in H line strengths….

“Spectral types” based on H lines strength: A B C D E F…

1901, Annie Jump Cannon > spectral classification

11 Spectral Classification

• Spectral classification system – Arranges star classifications by temperature • Hotter stars are O type • Cooler stars are M type • From hottest to coldest, they are • New Types: L and T O-B-A-F-G-K-M – Cooler than M – Mnemonics: “Oh, Be A Fine Girl/Guy, Kiss Me – Or: Only Bad Astronomers Forget Generally Known Mnemonics

A convenient tool for organizing stars

• In the previous unit, we saw that stars have different temperatures, and that a star’s luminosity depends on its temperature and diameter • The Hertzsprung-Russell diagram lets us look for trends in this relationship.

12 Stars come in all sizes…

• A star’s location on the HR diagram is given by its temperature (x-axis) and luminosity (y-axis) • We see that many stars are located on a diagonal line running from cool, dim stars to hot bright stars – The • Other stars are cooler and more luminous than main sequence stars – Must have large diameters – (Red and Blue) Giant stars • Some stars are hotter, yet less luminous than main sequence stars – Must have small diameters – stars • So what’s going on here?

The Mass-Luminosity Relation

13 Stars come in all sizes… • L vs T L b = 4"d 2

b=brightness, d=distance ! away 2.9 "106K # nm T = $

! on the Main Sequence

14 The Main-Sequence

High-mass starsLifetime 10 mpg of a StarLow-mass stars 60 mpg

• The length of time a star spends fusing hydrogen into helium is called its main sequence lifetime – Stars spend most of their lives on the main sequence – Lifetime depends on the star’s mass and luminosity – More luminous stars burn their energy more rapidly than less luminous stars. – High-mass stars are more luminous than low-mass stars – High mass stars are therefore shorter-lived! • Cooler, smaller red stars have been around for a long time • Hot, blue stars are relatively young.

A (temporary) new lease on life

• The triple- provides a new energy source for giant stars • Their temperatures increase temporarily, until the helium runs out • The stars cool, and expand once again • The end is near…

15 Evolution to phase

Fuel runs out Core temperature rises Core heats up more Core pressure drops Shell burning Shell burning grows stronger Gravity compresses Atmosphere expands and core Pressure puffs outer layers cools further • The star is expanding and cooling, so its luminosity increases while its temperature decreases • Position on the HR diagram shifts up and to the right…

16 Helium Fusion

• Normally, the core of a star is • The core of a red is not hot enough to fuse helium very dense, and can get to very – Electrostatic repulsion of the high temperatures two charged nuclei keeps them – If the temperature is high apart enough, helium fuses into Beryllium, and then fuses with another helium nuclei to form carbon.

Main Sequence Turn-off

What are we looking at? a) Stars of the same Main sequence mass? b) Stars of the same color? c) Stars of the same ? d) Stars of the same age?

17 Main Sequence Turn-off

What does the mass of the Main Sequence-Turn-off tell us? Main sequence a) The Mass of the cluster? b) The Age of the cluster? c) The Distance of the cluster? d) The Brightness of the cluster? Mass at turn-off

The Life-path of the Sun

18 Formation of

• As a red giant expands, it – This drag creates a cools high-speed stellar – Outer layers cool enough for carbon flakes to form wind! – Flakes are pushed outward – Flakes and gas form by radiation pressure – Flakes drag stellar gas a planetary nebula outward with them

White Dwarf Stars

• At the center of the planetary nebula lies the core of the star, a white dwarf – Degenerate material – Incredibly dense • Initially the surface temperature is around 25,000 K • Cools slowly, until it fades from sight.

19 The Lifespan of a Massive Star

Layers of Fusion Reactions

• As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace • Eventually the temperature climbs enough so that the helium begins to burn, fusing into Carbon. Hydrogen continues to burn in a shell around the helium core • Carbon is left behind until it too starts to fuse into heavier elements. • A nested shell-like structure forms. • Once iron forms in the core, the end is near…

20 Stellar Corpses

• A type II leaves behind the collapsed core of neutrons that started the explosion, a . • If the neutron star is massive enough, it can collapse, forming a black hole…

Azimuthal Radial •Most luminous EM events in velocity velocity the universe •Short (few sec) gamma ray burst •Longer optical afterglow •2 kinds (short and long)

pressure density •(1)Massive star death •(2)Collisions of dead stars

21 Supernova Remnant

• The supernova has left behind a rapidly expanding shell of heavy elements that were created in the explosion. • Gold, uranium and other heavies all originated in a supernova explosion!

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