Stars and Planets

Dr. Mary Urquhart

University of Texas at Dallas
CAST 2007 Presentation
 Activity 1:
Scale Model Solar System
Scale Factor of 1 to 10 billion

Based on Colorado Model Solar System, on the campus of University of

Colorado at Boulder (which was also the inspiration for the scale model solar
system on the National Mall in Washington D.C.
 Key Concepts (Part 1)
• All planets are smaller than the Sun.
• The Earth is a very small planet.
 Mercury

1
 Venus

2
 Earth

3
 Mars

4
 Jupiter

5
 Saturn

6
 Uranus

7
 Neptune

8
 Pluto?
(and its moon Charon)

9
The asteroid Ceres was once a planet, too. More on Pluto
and the other dwarf planets for middle school presented
on Friday (WS 2416) at 12:30 pm.
 Key Concepts (Part 2)
• The solar system is mainly empty space.
• The scale of the solar system is immense.
• The small inner planets (Mercury, Venus, Earth
and Mars) are much closer to the Sun than the
outer planets.
 Lesson 2: Sizes of Stars
Uses same 1 to 10 billion scale factor as the Scale Model Solar
System activity.
 A Hundred (or Two) Billion Stars

Our Sun is but one star out of more than 200 billion stars in the
Milky Way galaxy!

Aside: Why can’t the galaxy used in the above image be a real
picture of the Milky Way?
 Spectral Classes of Stars
The spectral class of a star is dependent the temperature of the star’s
photosphere. There are seven major spectral classes:

* O 30,000 - 60,000 K Blue-white stars

B 10,000 - 30,000 K Blue-white stars
A 7,500 - 10,000 K White stars
F 6,000 - 7,500 K Yellow-white
G 5,000 - 6,000 K Yellow stars (like the Sun)
K 3,500 - 5,000K Yellow-orange stars
M < 3,500 K Red stars
Newly adopted spectral classes of very low-mass stellar objects: L and T (these objects are considered
by many to be brown dwarfs- true failed stars, and have too little mass for hydrogen fusion to begin, but
do have the fusion of lithium and deuterium as a short-term energy source.
 Remembering Classes
Traditional:
Oh Be A Fine Girl, Kiss Me!

Or try:
Oh Be A Fine Guy, Kiss Me!

A great activity for students of all ages is to have them create

their own mnemonic phrases.
 Black Body Radiation
Temperature, and Color
Windows to the Universe has a
classroom friendly explanation of
black body radiation, temperature,
and color.

Go to:

http://www.windows.ucar.edu/tour
/link=/cool_stuff/tourstars_13.html

Image Source: http://www.windows.ucar.edu/tour/link=/cool_stuff/tourstars_13.html

Lesson 3: Distances of Stars
How Far Away are the Nearest Stars?
If we were to add the nearest star system, Alpha Centauri to our
scale model solar system, how big a space would we need?
Would it be on campus?
Near campus?
Across town?
In another city?
On the other side of the US?
In another country?
Around the world?
Alpha Centauri System

To scale for size, not distance.

Lesson 4: Star Birth
Orion Nebula: What do you see?
Emission Nebula are Stellar Nurseries

Eagle Nebula
Initial Mass Function

Not all stars are created equal!!

 Activity:
Probability and Star Formation
 Stellar Disks with Tails??

Image from: http://physics.hallym.ac.kr/education/stellar/hst/pubinfo/PR/94/24.html

Many of the disks around young stars in the Orion nebula are being
destroyed by the light and stellar winds of the hot O and B stars in
their stellar nursery, giving the disks the appearance of tails
pointing away from the cluster of massive stars.
 Protoplanetary Disks in Orion

More information at: http://hubblesite.org/newscenter/archive/1995/45/image/b

Internet Explorations
 Lesson 5: Lifetimes of Stars

The Pleiades is a famous cluster of young stars visible in the constellation Taurus. When most of the gas and
dust is gone from a stellar nursery, the young stars are in an open cluster. One day these star systems will
drift apart. Image source: http://antwrp.gsfc.nasa.gov/apod/ap021201.html

In Lifetimes of Stars, students make a scale model of

time, rather than of size or distance.
 Lifetimes of Stars
Our own star, the Sun, will “live” as a main sequence star for about
10 billion years. We are about halfway through its lifetime.
However, because its brightness increases as it builds up helium in
its core, the Earth will only be habitable for another couple billion
years.

To determine the lifetime of another star, take its mass, M,

relative to the mass of the Sun, and use the equation above. Then
you will have the lifetime of the star relative to our own Sun.
 Comparing Stellar Lifetimes to the
Geologic History of the Earth
Stars and Planets lesson 5: Lifetimes of Stars
Available at http://lyra.colorado.edu/mary/stars

How do the lifetimes of the different classes of main sequence stars

compare with the history of our planet? Why are astronomers more
interested in finding Earth-like planets around stars of particular
classes? How long does it take before a planet would have
“interesting” life, assuming the history of life on Earth is typical?
 Star Birth and Death
The most massive stars die in dramatic explosions called
supernova. These stars also spend the shortest time on the main
sequence. This means that many massive stars die while still
embedded in the same stellar nurseries in which they were born.
Supernovae can also lead to more star formation!
 Open Clusters

Image of the Pleiades: http://www.cv.nrao.edu/~pmurphy/images/astro/ Image from http://www.astropix.com/HTML/B_WINTER/M46.HTM

After several hundred million years, and perhaps several generations of

high mass stars, the remaining gas and dust in a stellar nursery starts to
dissipate, leaving an open cluster of young stars. These stars will
eventually drift apart and become independent stars or star systems in
the galaxy.
 Lesson 6: Death of Stars

Crab Nebula, a supernova remnant Image source: http://antwrp.gsfc.nasa.gov/apod/ap991122.html

In this activity, we return back to scale modeling of

size with the 1 to 10 billion scale factor
to look at dying stars and stellar remnants
 Stellar “Evolution”
Stars, like people, change as they age. While on the main
sequence, a star’s color is directly related to its mass. Once a
star runs out of hydrogen fuel in its core, however, everything
changes! Stars will “evolve” off the main sequence with great
changes in size, surface temperature, and luminosity. The final
fate of an isolated star will be very dependent upon its initial
mass.

At each stage of the life of a star, gravity plays a leading role.

 Class, Mass, and Luminosity
Once a star leaves the main sequence, its spectral class
(dependent on surface temperature) has little to do with mass.
Fortunately, evolved stars can easily be distinguished from main
sequence stars by their luminosities (intrinsic brightness).
Hertzsprung-Russell Diagram
Leaving the Main Sequence
Online Lessons and Activities for HR Diagrams

Try http://www.smv.org/jims/hr/hrEX.htm to construct your own H-R

diagram. How did you do?

Then go to http://www.smv.org/jims/unit.htm to see the online lesson

How Hot is that Star? From the beginning.

For an advanced high school interactive online lesson on HR

diagrams, visit http://skyserver.sdss.org/dr1/en/proj/advanced/hr/.
This is only one of many advanced and “beginner” lessons available
from the Sloan Digital Sky Survey.

Jewels of the Night, is a High School lesson requiring access to a color

printer. See http://www.noao.edu/education/jewels/home.html.
 Red Giants and Supergiants

These stars generally produce more energy per unit time than they
did when on the main sequence, resulting in a very bright, but cool,
M class star.
Scale Model Red Giants and Supergiants

Complete Table 1 in Stars and Planets lesson 5, Death of Stars

How do your scaled sizes for red giants and supergiants

compare with the sizes of main sequence stars?

What does this tell you about the average density of giant
stars?
 How Big is a White Dwarf?
Go to Table 2 in Stars and Planets lesson 5, Death of Stars

Compare the size of the white dwarf to the size and distances
of planets in the solar system. Which object is closest to the
size of a white dwarf?

What object could you use to represent a white dwarf?

Core of an Evolved Supergiant

Image Source: http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit2/himass.html

Si “burning” stage only lasts about 1 day!

 More Illustrations and Information

http://chandra.harvard.edu/resources/illustrations/supernova.html

http://www.solstation.com/x-objects/crab-neb.htm
 Result: Supernova!

Image Source: http://rst.gsfc.nasa.gov/Sect20/A10.html

A supernova can be brighter than the light from an entire galaxy

of more than 100 billion stars! Only rare, massive stars are
massive enough to reach the iron limit and go supernova.
From Supernova to Neutron Star

Illustration Credit: CXC/S (NASA), Lee, from http://chandra.harvard.edu/resources/illustrations/supernova.html

 Crab Nebula
The result of
a supernova
recorded by
Chinese
astronomers
in 1054.

Image from http://www.noao.edu/image_gallery/html/im0565.html

 What is left behind?
The core of the massive star continues to collapse. How much mass
is left determines its final fate.
 More Size Comparisons
Complete Table 2 in Stars and Planets lesson 5, Death of Stars

Why can’t neutron stars and black holes be represented on a

model with our 1 to 10 billion scale factor?

How would you help students understand the true size of a

neutron star and the apparent size of a black hole?
Neutron Stars: So Dense, So Small!

Neutron stars are

similar in size to
cities, as shown by
this illustration of
a neutron star
compared with
Manhattan Island
in N.Y.

Image Source: http://www.pas.rochester.edu/~afrank/A105/LectureXI/LectureXI.html

 Density in the Extreme

Image Credit: Ann Feild (STScI)

Black holes contain less matter than the stars they were born from,
and so have less gravity at a given distance. But, they are so small
and dense, that other matter can come very close to their centers, and
there the gravity is strong enough that even light can’t escape.
Gravity in the Extreme: Black Holes
The gravitation pull of black holes is so great only because
they are so unimaginably dense.

⇒ If the Earth became a black hole it would have a

“diameter” of 0.01 m (1 cm).

At a given distance from the center of a black hole, its

gravitational pull is no greater than any other object with the
same mass.

Want to know what it is like to fall into a black hole? Check

out http://casa.colorado.edu/~ajsh/schw.shtml.
Lesson 7: Planet Hunting

Part 1: Finding Planets

Part 2: New Discoveries
Finding Planets: Why Is It So Hard?
 Finding Planets: Why Is It So Hard?
Problem 1: The sizes of planets and distances of stars.

“Trying to see the Earth from Alpha Centauri is like trying

to see a candy sprinkle on a donut in New York when you
are standing in San Francisco.” - from Planet Hunting

The discoveries of Uranus, Neptune, and the dwarf planets all

required telescopes!
Finding Planets: Why Is It So Hard?
Problem 2: Stars are very bright and shine with their own
light. Planets are very dim and shine by reflected light.

We can only see stars because of their brightness. All but

two are no more than pinpoints of light in the sky, even with
the best telescopes!

Pluto, as seen in a
three minute exposure
on a 24” telescope at
Swathmore College
in PA.

Image from http://www.science.widener.edu/~schultz/sproul10.html

 Radial Velocity Measurement or
Doppler Spectroscopy

Image from http://www.hao.ucar.edu/public/research/stare/search.html

Any two objects in the Universe exert a gravitational pull on each

other, including stars and planets. Astronomers can use a change in
the frequency of light received from a star caused by the “wobble”
of a star with a planet pulling on it to detect the planet. This works
best for detecting massive planets close to their parent star.
Illustration of Doppler Shift Due to
Stellar Wobble

Illustration by Geoff Marcy from http://cannon.sfsu.edu/~gmarcy/planetsearch/tech/Stellar_Wobble.gif

 Astrometry

Image from http://www.hao.ucar.edu/public/research/stare/search.html

http://sim.jpl.nasa.gov/science/images/my_wobble.gif

By measuring the position of a star with great precision, it is

possible to directly detect the wobble of a star. This works best for
detecting massive planets in tight orbits around stars in the solar
neighborhood.
 Transits and Photometry
Mercury→ A planet passing between
us and its star will also
decrease the amount of
light we can see from the
star…but not by much.

See more on the

November 1999 transit of
the planet Mercury at
http://lyra.colorado.edu/public/past
performances/mercury.html.

Image from: http://www.astrochem.org/bigsun.JPG

Scale Models of Planetary Systems
Using the same 1 to 10 billion scale factor we used for the
Scale Model Solar System and our everyday objects to
represent stars and planets, we can model the extrasolar
planetary systems found around other stars.

An important point is that adding mass to a Jupiter-type planet

does little to change its diameter. The planets simply become
more dense with added mass until fusion beings.
 Kepler

Kepler is a NASA Discovery mission with the primary goal of

finding Earth-like (terrestrial) planets around other stars. It is
scheduled to launch in 2008 and will use the transit method from
space.

Three transits of the same planet detected by Kepler will be

treated as a confirmed detection. Kepler will be able to
determine the orbital time of a detected planet, the size of the
planet, and an estimate of its surface temperature.
Learn more at http://www.kepler.arc.nasa.gov/
 Terrestrial Planet Finder
Terrestrial Planet Finder is a space-based mission, still under
development at NASA, that has the goals of not only detecting
Earth-like planets around other stars, but imaging those planets and
returning spectra of their atmospheres to look for signatures of life.

Terrestrial Planet Finder will also be able to

closely observe the planetary formation
process in stellar nurseries, as well as targets
of more general astrophysical interest.

Learn more at:

http://planetquest.jpl.nasa.gov/TPF/tpf_index.html
Lesson 8: Gaggle of Galaxies
How do sizes and
distances of galaxies
compare with stars?

What different types of

galaxies can you see in
Hubble Ultra Deep
Field?

How would you

classify the galaxies in
this image?

Why are some galaxies

red in the image, and
how do they relate to
the Big Bang?
The Milky Way - A Barred Spiral Galaxy

Image from
http://starchild.gsfc.nasa.gov/docs/StarChild
/universe_level1/milky_way.html

The image on the left is an artist conception based on data from the
Spitzer Space Telescope. The image on the left is really of our
galaxy and was taken using infrared and microwave light. Because
we are in the galactic disk, we see our own galaxy as a thin line that
is thicker in the middle, where the central bulge is located.
 The Milky Way on Edge

This image of the Milky Way, taken in Infrared by the Two-Micron All
Sky Survey (http://www.ipac.caltech.edu/2mass/gallery/) shows the
clouds of dust in the disk of the galaxy that obscure our view of many
Milky Way stars. The disk of our galaxy is roughly 100,000 l.y. across,
but has a maximum thickness of “only” a few 1,000 light years.
 Andromeda and M33

Image Source: http://www.astrophoto.com/M33.htm

Image Source: http://isaac.exploratorium.edu/~pauld/activities/astronomy/cityuniversesizeillo.html

The largest visible galaxy in the local group (200,000 ly in diameter),

Andromeda has the most stars of any local group galaxy, about 300
billion compared with our 200 billion stars. M33, by contrast is a
small spiral galaxy with a mere 25 billion solar masses and a diameter
of 50,000 ly. M 33 is also known as the “Triangulum Galaxy”.
 A Gaggle of
Galaxies
What types of galaxies can
you see in this Hubble
mosaic?
 Major Types of Galaxies

http://www.seds.org/messier/m/m087.html

Most galaxies are spiral, elliptical

(giant or dwarf), or irregular
galaxies. At right is the Large
Magellanic Cloud, an irregular
galaxy orbiting the Milky Way.
http://www.seds.org/messier/xtra/ngc/lmc.html
 Strange Galaxies

Some galaxies are difficult to classify,

like the Ring Galaxy, which is an odd
spiral. The Sombrero Galaxy is a
normal spiral, but looks odd in our side
view. Some strange galaxies are the
result of collisions with other galaxies.
These collisions don’t mean stars hit
one another…they just get mixed up.
Red Shift and an Expanding Universe
As the universe expands, light
expands with it, increasing in
wavelength.

Image from
http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap19.html

Red light is the longest

wavelength of visible light, so
this increase in wavelength is
called redshift. The earliest
visible light emitted in the
Universe now is so large in
wavelength that it is microwave
Image from: http://saturn.jpl.nasa.gov/mission/nav-uplink.cfm radiation.
 Hubble’s Law (For the Teacher)
In the 1920’s Edwin Hubble not only showed that many “nebula”
were in fact other galaxies, but that galaxies appear to be speeding
away from one another. The further away a galaxy was (measured
with standard candles), the faster it seemed to move away from us!
velocity = Ho x distance, where Ho is a constant
A redshift can be described by the quantity z, where
1 + z = λobserved/λrest

As light travels through space-time, it is

expanded along with the rest of the
universe, creating longer and longer
wavelengths (redshift). The longer the
amount of time the light has traveled
through space, the greater the redshift.
Image from http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap19.html
 Historical Views
In the second century A.D., the Greek astronomer and
mathematician Ptolemy built a secular cosmology
with the Earth at the center of the Universe. This
cosmology of the nature of the Universe was adopted
by the Catholic church. Image From http://www-gap.dcs.stand.ac.uk/~history/Mathematicians/Ptolemy.html

Copernican Revolution: Led by Copernicus, Galileo, and Kepler, the

Copernican Revolution gave science a Sun-centered view of the
solar system, with the Sun as one of many stars in the Galaxy.

Einstein, like many astronomers of his day, initially believed that the
Universe is static and has always been as it is today.
 Expanding Universe
Objects in an expanding Universe don’t just appear to speed away
from one another…the distances between them are increasing, just
like the distance between raisins increases in rising raisin bread!

Image from http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap19.html

A raisin in the bread sees all other raisins move away from it. To the
raisin, it looks like it is at the center of the expansion…but it really
isn’t. There is no measurable center of the Universe!
 Not Everything Expands!
Objects bound together by fundamental forces…people,
asteroids, planets, stars, galaxies, etc. don’t expand with the
Universe. These objects stay the same size. The distance
between objects not bound to one another does increase with the
expansion of the Universe.

Image from http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap19.html

Think of glitter on the surface of an expanding balloon, or

raisins in bread as galaxies. These objects stay the same size,
even though when the balloon or bread expands.
Explore the Expanding Universe in the Classroom
The Universe activity from the Sloan Digital Sky Survey
http://skyserver.sdss.org/dr1/en/proj/basic/universe/default.asp

Middle School Student Friendly Center of the Universe Discussion from JPL
http://spaceplace.jpl.nasa.gov/phonedrmarc/jun2003.html

No Edge, No Center - Exploring the Shape of Our Universe is a high school activity
with teacher guides from http://universe.sonoma.edu/activities/no_edges.html

A Ballooning Universe in The Universe at Your Fingertips (grades 8+) from

Astronomical Society of the Pacific (http://www.astrosociety.org).

The Expanding Universe in The Universe at Your Fingertips (grades 8+)

Visualization of the Expansion of Space in The Universe at Your Fingertips

(grades 8+)