Lecture 8 Starlight And Atoms

Announcements  Homework 5 – Due Monday, Feb 26 Unit 23: RQ 1, P 2, TY 3Unit 23: RQ 1, P 2, TY 3 Unit 24: RQ 1, 2, TY 2Unit 24: RQ 1, 2, TY 2 Unit 25: RQ 1, P 2Unit 25: RQ 1, P 2 Unit 55: RQ 3, P 4Unit 55: RQ 3, P 4

The Basic Idea  Stars produce light.  Star light tells us a lot about the star it comes from.  NEED to use star light to study stars because the material from a star has never been collected for study!

Where Does Star Light Come From?  The light we detect from a star comes from the star’s photosphere.  The photosphere consists of the glowing gases in the outer “surface” of a star.

How Can A Cloud Have A Surface?  A photosphere is made of a thin gas, so how can it be a surface? A photosphere isn’t a solid surface like the Earth’s surface, BUT… It glows so brightly you can’t see through it to the gases below. It’s a surface in the sense you can’t see under it, just like the surface of the Earth.

Why Does A Photosphere Glow?  A photosphere glows because it is very hot!  The hotter something is, the more light it produces AND the higher energy (on average) that light becomes.  Makes hotter objects look BRIGHTER and BLUER than cooler ones.

What is Heat?  To understand why hot things glow, we need to understand what heat is.  Heat is a way to measure how quickly and violently the atoms or molecules in a substance are vibrating.  These vibrations cause the atoms and molecules to collide.

Heat Creates “Free Electrons”  In any substance some of the atoms and molecules vibrate faster than others.  The hotter something is, the more violently its atoms and molecules vibrate on average.  Some collisions between atoms/molecules are violent enough to “bump” electrons off of their atoms. These are called free electrons. Colliding atoms sometimes create… …a free electron

More Heat Means More Free Electrons  Since a hotter substance has faster moving atoms on average: Collisions are more frequent.Collisions are more frequent. Collision are, on average, more violent.Collision are, on average, more violent. More collisions are able to create free electrons.More collisions are able to create free electrons.  So the hotter something gets, the more free electrons it has. Cool gas … not many free electrons. Hot gas … lots more free electrons.

Free Electrons Also Collide With Atoms  In a hot gas, the free electrons also collide with each other, and the other atoms in the gas…  These collisions accelerate the electrons (change their speed and direction).

The Light Comes From Accelerated Electrons!  When an electron is accelerated it gives off light. The more the electron is accelerated the more energy the emitted light has.The more the electron is accelerated the more energy the emitted light has. For light, higher energy makes bluer light.For light, higher energy makes bluer light.  Very low energy light is invisible: infrared, microwaves, and even radio waves.  Low energy light looks red to us.  Light with more energy than red light looks orange. Even more energy makes it look yellow, then green, then blue, then violet.  Light with even more energy than violet is ultraviolet (and is invisible to us).  X-rays and gamma rays have even more energy than ultraviolet.

More Accelerated Electrons Means More Light!  REMEMBER: The hotter something is, the more free electrons it has.  So there are MORE collisions, and we make MORE light.  CONCLUSION: A hotter substance will produce (on average):A hotter substance will produce (on average):  BLUER light (higher energy light)  MORE light (more photons because there are more free electrons to make light)

Blackbody Radiation  The light (EM radiation) produced by free electrons in a hot object is called blackbody radiation.  “Hot” is relative! Anything above absolute zero produces some blackbody radiation.

The Radiation Laws  There are two natural laws (equations) that describe Blackbody Radiation.  How Bright Is The Light? Stefan-Boltzmann Law:Stefan-Boltzmann Law: E =  T 4 E = amount of light energy produced per square meter from the object’s surface each second (in Watts/m 2 )E = amount of light energy produced per square meter from the object’s surface each second (in Watts/m 2 ) T = temperature of the surface (in Kelvin)T = temperature of the surface (in Kelvin)  = 5.67 × 10 -8 = 5.67 × 10 -8

The Radiation Laws  What Color Is The Light? Actually, light of all colors is produced, but the most common color of light is given by Wein’s Law:Actually, light of all colors is produced, but the most common color of light is given by Wein’s Law: max = 3,000,000 / T max = 3,000,000 / T max is the wavelength of the most common color of light (more photons are this wavelength than any other). The units are nanometers! max is the wavelength of the most common color of light (more photons are this wavelength than any other). The units are nanometers! T is the temperature (again, in Kelvin)T is the temperature (again, in Kelvin)

What is this “Kelvin”?  There is a temperature that is so cold that atoms and molecules stop moving completely: absolute zero. At -273ºC (or -459ºF)At -273ºC (or -459ºF)  Lord Kelvin invented a new temperature scale, based on the Celsius scale, where the zero point is absolute zero.

The Kelvin Scale  On the Kelvin scale, there are no negative temperatures (no such thing as a temperature below absolute zero).  Subtract 273 from the Kelvin temperature to get the Celsius temperature.  Unlike common temperature scales, which are ratios based on two fixed points, the Kelvin is an absolute unit of measure and so is not expressed in “degrees”.

Using The Radiation Laws  The sun’s surface temperature is 5,800 K. How much light energy comes from each square meter of the sun’s surface each second?  Solution: E = ( = 5.67 × )(5,800 K) 4 E = 64,164,532 W/m 2 (The W stands for “Watts”)

Using The Radiation Laws  What is the most common color of light the sun produces?  Solution: max = 3,000,000 / (5,800 K) max = 3,000,000 / (5,800 K) max = 517 nm max = 517 nm (This is actually green light, but it is mixed in with enough yellow that our eyes see this color as yellowish)

Daily Grade 8 – Question 1  The wavelength of maximum intensity that is emitted by a black body is A.proportional to temperature. B.inversely proportional to temperature. C.proportional to temperature to the fourth power. D.inversely proportional to temperature to the fourth power.

What color are the stars?  We can use Wein’s Law to figure this one out…  First, all stars produce light at all colors! And we see all colors together as white!  But, the fact that more photons of one color are produced than any other gives the white light from the star a tinge of color.

What color are the stars?  The coolest stars are around 3,000 K max = 1,000 nm max = 1,000 nm That’s infrared light, which means that in visible light the star will produce more red photons than any other color.That’s infrared light, which means that in visible light the star will produce more red photons than any other color. The star will look reddish-white.The star will look reddish-white.

What color are the stars?  The sun has a temperature of about 5,800 K. max = 517 nm max = 517 nm Tinges the sun’s white light yellow.Tinges the sun’s white light yellow. The star will look yellow-white.The star will look yellow-white.

What color are the stars?  The hottest stars have temperatures over 40,000 K. max = 75 nm max = 75 nm This is ultraviolet light.This is ultraviolet light. In visible light the star produces more blue light than any other color.In visible light the star produces more blue light than any other color. Makes the star look blue-white.Makes the star look blue-white.

The Color Index  So very hot stars are bluish, and very cool stars are reddish.  But how do we quantitatively measure a star’s color.  In astronomy, use the color index.

The Color Index  Procedure: Take a picture of a star through two different color filters.Take a picture of a star through two different color filters.  Two common ones to use in astronomy is a blue (B) filter and a green “visual” (V) filter. Measure the magnitude of the star in each filter (i.e. how bright it looks in each filter).Measure the magnitude of the star in each filter (i.e. how bright it looks in each filter). Take the two magnitudes and subtract them:Take the two magnitudes and subtract them: B – V This number is called the “B-V Color Index” B band V band

The Color Index  Remember, the brighter a star is the smaller its magnitude!  A blue star: Looks brighter through the B filter than the V one.Looks brighter through the B filter than the V one. B-magnitude is smaller than V-magnitude.B-magnitude is smaller than V-magnitude. B – V = small number – large numberB – V = small number – large number = negative number Blue stars have a negative color indexBlue stars have a negative color index

The Color Index  Remember, the brighter a star is the smaller its magnitude!  A red star: Looks brighter through the V filter than the B one.Looks brighter through the V filter than the B one. V-magnitude is smaller than B-magnitude.V-magnitude is smaller than B-magnitude. B – V = large number – small numberB – V = large number – small number = positive number Red stars have a positive color index.Red stars have a positive color index.

Daily Grade 8 – Question 2  The B - V color index of a star indicates its A.total mass. B.radius. C.chemical composition. D.surface temperature.

A Primer on Atoms  An atom contains: A positive nucleus composed of two types of particles:A positive nucleus composed of two types of particles:  Protons – actually have the positive charge.  Neutrons – no charge, but same mass as protons.  Contains most of the atom’s mass. Electrons (negative charge) that orbit the nucleus.Electrons (negative charge) that orbit the nucleus.  Electrons have very little mass compared to protons and neutrons.

Daily Grade 8 – Question 3  Which subatomic particle has a negative charge? A.The electron. B.The proton. C.The neutron. D.Both the neutron and the proton.

Atomic Density If you could fill a teaspoon just with material as dense as the matter in an atomic nucleus, it would weigh ~ 2 billion tons!!

Different Kinds of Atoms The kind of atom depends on the number of protons in the nucleus. Different numbers of neutrons ↔ different isotopes  Most abundant: Hydrogen (H), with one proton (+ 1 electron).  Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.).

Daily Grade 8 – Question 4  Which of the following is true of an atomic nucleus? A.It contains more than 99.9% of an atom's mass. B.It contains all of an atom's positive charge. C.It contains no electrons. D.All of the above.

Next Time  Read Units 25 and 55 (yes, I mean 55, way towards the back. I did say we’d be jumping around in this book).