Stars: Intro & Classification...Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015...

Astronomy 1 — Elementary Astronomy LA Mission College

Spring F2015

Stars: Intro & Classification

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015

Quotes & Cartoon of the Day

“The wonder is, not that the field of stars of so vast, but that man has measured it.”

— Anatole France, The Garden of Epicurus, 189

Announcements

• SS Homework posted, due 11/19

• Midterm — will debrief THURSDAY

• Anybody see the….

Last Class

• Solar System Topics

This Class

• Intro to Stars

• Temperature, Color & Size

• Stellar Classification

• Intro to the HR Diagram

• LT HR Diagram

Spring F2015

Stars, Temperature and Color

Recall What a Star Is

• A sphere of hot gas

• mostly hydrogen & helium

• Interior hot enough to undergo nuclear fusion

• most commonly H —> He

• above 107 K = 10 million K (18 million °F)

• Held together by gravity

Fundamental Properties of Stars

• TEMPERATURE

• color, spectral properties

• LUMINOSITY

• inherent brightness

• amount of energy generated in the star and released as electromagnetic radiation

• SIZE

• Radius & Mass

• DISTANCE

• CHEMICAL COMPOSITION

Luminosity

• Luminosity is a measure of the rate at which a star puts out energy.

• Larger luminosity — appears brighter

• measured in J/s or “watts”

• sometimes relative to the Sun, in “solar luminosities” L☉

• L stands for luminosity, ☉stands for Sun

• The Sun has a luminosity of 3.846×1026 W

HOW HOT IS THAT STAR?

Recall: Why Stars Shine

• Visible Sun is a hot layer of gas

• about 5800 K (5525°C, 9980°F)

• not as hot as the center!

• Glows due to hot material

• Other stars have similar structure, range of surface temperature

Blackbody Radiation

• white light spectrum is continuous

• no gaps between colors

• the hotter you heat something, the bluer it appears

• Thermal (heat related) radiation from (relatively) hot bulk matter

• This is called blackbody radiation

Light diagnoses Temperature

feeling warm

faint reddish glow

brighter red glow

bright orange

very bright yellow

extremely bright white

bluish whitehttp://i302.photobucket.com/albums/nn114/fiddlegirl89_photos/Dave/DSC00596-1.jpg

Colder

Warmer

Spectrum of the Sun

• Stars emit light as blackbodies to a large extent.

Shape of Blackbody Spectrum

Temperatue affects Color & Luminosity

• Hotter = greater luminosity

• appears brighter

• peak of curve higher

• technically greater luminosity per unit surface area

• Hotter = bluer

• peak of curve more to left

• more blue mixed in with red light

Let’s Practice

A lump of lead is heated to a high temperature. Another lump of lead that is twice as large is heated to a lower temperature. Which lump of material appears bluer?

A. The cooler lump appears bluer

B. The hotter lump appears bluer

C. Both lumps appear the same color

D. Cannot tell which lump appears bluer

When something is “red hot”, it is hotter than something that is

A. blue hot

B. white hot

C. neither of these

D. both of these

SIZE, COLOR AND TEMPERATURE

Recall

• Hotter -> Brighter

• Hotter -> Bluer

• Bigger is also brighter!

• a star with a larger diameter has a greater luminosity

http://www.skinnerscience.com/Year%2012,13/astrop3.jpg

Analogy

• LED spotlight w/60 LEDs

• Total output ~1000 lumens

• comparable to 100-watt bulb

• each LED 1/60 of 1000 lumens

• ~16.7 lumens

• surface T gives you “per-LED” luminosity not total for star

Stephan-Boltzman Law

is brighter than

could be brighter than

Let’s Practice

Star Yoda is a small blue star and star Chewbacca is a large red star. Which is brighter?

(in terms of light output, not intelligence...)

A. Yoda

B. Chewbacca

C. They have the same brightness.

D. It’s not possible to tell.

Use the graph at right to determine which of the two stars (A or C) has a higher luminosity?

A. Star A

B. Star C

C. The two stars have the same luminosity.

D. It is not possible to determine this.

Blackbody Radiation – Instructor’s Guide

Third Edition

2) Use the graph at right to determine which of the following best describes how Star A would appear as compared with Star B?

a) Star A would appear more red than Star B. b) Both stars would appear more red than blue. c) Both stars would appear more blue than red. d) Star A would appear more blue than Star B. e) None of the above.

3) Use the graph at right to determine which of the two stars (A or B) emits light with the longer peak wavelength? a) Star A b) Star B c) Both stars peak emissions are at the same wavelength. d) None of the above are possible.

4) Use the graph at right to determine which of the two stars (A

or D) gives off more green light? a) Star A b) Star D c) They both give off the same amount of green light.

or C) is at a higher temperature? a) Star A b) Star C c) The two stars have the same temperature. d) It is not possible to determine this.

6) Use the graph at right to determine how the size of Star A compares to the size of Star C. a) Star A is smaller than Star C. b) Star A is larger than Star C. c) The stars are the same size. d) It is not possible to determine this.

Use the graph at right to determine which of the following best describes how Star A would appear as compared with Star B?

A. Star A would appear more red than Star B.

B. Both stars would appear the same color.

C. Star A would appear more blue than Star B.

Third Edition

Use the graph at right to determine which of the two stars (A or B) is at a higher temperature?

A. Star A is at a higher temperature than Star B.

B. Both stars have the same temperature.

C. Star B is at a higher temperature than Star A.

Third Edition

Use the graph at right to determine which of the two stars (A or C) is at a higher temperature?

A. Star A

B. Star C

C. The two stars have the same temperature.

D. It is not possible to determine this.

Third Edition

Use the graph at right to determine how the size of Star A compares to the size of Star C.

A. Star A is smaller than Star C.

B. Star A is larger than Star C.

C. The stars are the same size.

D. It is not possible to determine this

Third Edition

The graph at right shows the blackbody spectra for three different stars. Which of the stars is at the highest temperature?

A. Star A

B. Star B

C. Star C

Blackbody Radiation – Instructor’s Guide 67

Third Edition

INTRODUCTION

Prerequisite Knowledge

• Basic familiarity with the relationships between temperature, wavelength, and spectral curves

• Know that electromagnetic waves can have a large range of possible wavelengths • Know that different types of electromagnetic waves have different wavelengths • Know that visible light is a type of electromagnetic wave

• Describe an object’s appearance given its blackbody curve • Infer a star’s color and temperature from its blackbody curve • Infer relative sizes of stars, when possible, from differences in their blackbody curves

Pre-activity Question

1) The graph at right shows the

blackbody spectra for three different stars. Which of the stars is at the highest temperature? a) Star A b) Star B c) Star C

TUTORIAL GUIDE 1) [ Red ]

Students have difficulty relating light curves to physical appearance and, in particular, struggle with the meaning of the word spectrum. This question introduces the concept of relative intensity.

2) [ Violet, indigo, green, yellow, and red (i.e., all the colors except blue and orange). ]

We intentionally do not suggest that there is a simple mechanism for accomplishing this and we avoid the concept of filters.

3) [ Figure a. ] 4) [ Blue and orange ]

Spring F2015

Spectral Classification

PRIMER ON SPECTRAL LINES

Spectroscopy

• Spectroscope or spectrometer — astronomical instrument that breaks light into its component wavelengths

• Where light is & is not present provides information

• what an object is made of

• its physical properties

http://www.ibsen.dk/technology/images/design/modules/spectrometer-sketch.jpg

Blackbody Spectrum

• continuous light spread across all colors

• a continuous or continuum spectrum

Emission Spectrumn

• light is present only in selected parts of the spectrum

• emission lines

• bright bands on a black background

Absorption Spectrum

• light is missing only in selected parts of the spectrum

• absorption lines

• dark bands on a blackbody/continuous spectrum

Atomic Structure and Energy Levels

• Electrons in an atom (this is Hydrogen) can be modeled as being in specific “shells” or orbits around the proton.

• Each shell has a specific energy level

• Further out & higher numbered shells have a higher energy

http://odin.physastro.mnsu.edu/~eskridge/astr101/kauf5_20.JPG

Atomic Transitions

• Electrons “prefer” to be in lower energy states

• can be kicked into a higher energy by absorbing a photon with the right energy.

• right energy , right wavelength

• E=hc/λ

Atomic Transitions

• Electrons “prefer” to be in lower energy states

• Drops back down by spitting out a photon with the right energy

• right energy, right wavelength

• E=hc/λ

Emission Spectrum

• This is a Hydrogen Emission spectrum

• Each line is due to a specific transition of electrons from a higher state to a lower state

• This pattern is specific to hydrogen, like a fingerprint

• Each element has a characteristic spectrum

Stellar Spectra (visible light portion)

Spectral Class

• Recall red stars are cooler than blue stars

• visual color isn’t very precise

• Astronomers turned to spectral properties to classify stars

• detailed classification based on line pattern

• Various schemes were tried

PICKERING’S HAREM &

THE HARVARD SPECTRAL CLASSIFICATION SCHEME

From Cosmos

• http://www.amazon.com/gp/product/B00K0CDNOI/ref=avod_yvl_watch_now

• From “Sisters of the Sun” Episode 8

• 7:45 to ~16:00

The Harvard Computers

• The director of the Harvard Observatory from 1877 to 1919, Edward Charles Pickering hired women to process astronomical data.

• They were cheaper than men

• earned less than a clerical worker

• Willamina Fleming had been his maid

The Harvard Computers

• "Pickering's Harem" or “the Harvard Computers” included several now-famous astronomers

• Annie Jump Cannon,

• Henrietta Swan Leavitt

• Antonia Maury

Modern Classification

• Harvard spectral sequence

• Developed by Annie Jump Cannon

• Characteristic absorption lines determine stellar class

• Note: in Astronomy “metal” means anything “heavier” than He

Modern Classification

• Cecilia Payne (Payne-Gaposchkin) discovered this was actually a temperature sequence

• More precise than BB peak or color

Spectral Classification

• From hot to cool:

• O B A F G K M

• Each spectral class is further divided into 10 ranges

• according to temperature. 0 = hot, 9 = cool

• therefore O0 is the very hottest, and O9 is slightly hotter than B0 and M9 stars are very cool

• This is still a temperature-based classification

Let’s Practice

Star Rue is Type K, Star Peeta is Type B, Star Katniss is Type F and Star Primrose is Type M.

Which star is cooler than Rue?

A. Peeta

B. Katniss

C. Primrose

D. None of them

You observe a very bright, bluish star. It’s spectral classification is most likely ____.

D. More information is needed to determine this

H-R DIAGRAM

Spectral Class isn’t everything

• Spectral Class is not sufficient to uniquely identify a type of star

• The supergiant Arcturus and the red dwarf Proxima Centauri are both Type M & 3500 K

• They are definitely not identical!

Hertzsprung and Russell

• In 1911 Danish astronomer, Ejnar Hertzsprung, plotted the absolute magnitude of stars against their color

• Independently in 1913 American astronomer Henry Norris Russell plotted spectral class against absolute magnitude

• showed that the relationship between temperature and luminosity of a star was not random

http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_hrintro.html

The H-R Diagram

• H-R diagram plots Color and/or Temperature against Luminosity and/or Absolute magnitude

• Any data plotted like this is an H-R diagram, as is a theoretical version

• The H-R diagram is one of the most important tools in Astronomy

The Main Sequence

• The long strip from upper left to lower right is called the “Main Sequence.” (MS)

• Stars spend most of their existence on the MS

• 91% of nearby stars are MS stars.

• MS stars are fusing H into He in their cores.

Giants and Dwarfs

WARM-UP QUESTION

Star A has an absolute magnitude of -8.1 and belongs to spectral class B8. Star B has an absolute magnitude of 11.2 and also belongs to spectral class B8. Which star has the higher temperature?

A. Star A

B. Star B

C. They have the same temperature.

D. There is not enough information to determine which star is hotter.

LECTURE-TUTORIAL ON THE H-R DIAGRAM

Star A has an absolute magnitude of -8.1 and belongs to spectral class B8. Star B has an absolute magnitude of 11.2 and also belongs to spectral class B8. Which star has the higher temperature?

A. Star A

B. Star B

C. They have the same temperature.

D. There is not enough information to determine which star is hotter.

Let’s Practice

A red giant of spectral type K9 and a red main sequence star of the same spectral type have the same _____.

A. luminosity

B. temperature

C. absolute magnitude

What Type of Star is Aldebaran?

A. Red Giant

B. Main Sequence

C. Supergiant

D. White dwarf

What Type of Star is Vega?

A. Red Giant

B. Main Sequence

C. Red Supergiant

D. White dwarf

WRAP-UP

Topic for Next Class

• Stellar Evolution

• Measuring Distances (time permitting)

Reading Assignment

• Astro:8

• Astropedia:13

Homework

• HW SS Posted, Due 11/19

Stars: Intro & Classification...Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015...

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