Transcript Option E: Astrophysics

OPTION E:
ASTROPHYSICS
M13/4/PHYSI/SP3/ENG/TZ1/XX
ASTEROIDS
1(a). State the nature of an asteroid.
minor planet / rocky/icy/metallic body;
Asteroids are small, airless rocky worlds revolving
around the sun that are too small to be called
planets. They are also known as planetoids or minor
planets. In total, the mass of all the asteroids is less
than that of Earth's moon. But despite their size,
asteroids can be dangerous. Many have hit Earth in
the past, and more will crash into our planet in the
future. That's one reason scientists study asteroids and
are eager to learn more about their numbers, orbits
and physical characteristics. If an asteroid is headed
our way, we want to know that.
• Most asteroids lie in a vast ring between the orbits
of Mars and Jupiter. This main asteroid belt holds more
than 200 asteroids larger than 60 miles (100 kilometers) in
diameter. Scientists estimate the asteroid belt also
contains more than 750,000 asteroids larger than threefifths of a mile (1 km) in diameter and millions of smaller
ones. Not everything in the main belt is an asteroid — for
instance, comets have recently been discovered there,
and Ceres, once thought of only as an asteroid, is now
also considered a dwarf planet.
• Many asteroids lie outside the main belt. For instance, a
number of asteroids called Trojans lie along Jupiter's
orbital path. Three groups — Atens, Amors, and Apollos
— known as near-Earth asteroids orbit in the inner solar
system and sometimes cross the path of Mars and Earth.
FORMATION
Asteroids are leftovers from the formation of our solar
system about 4.6 billion years ago. Early on, the birth
of Jupiter prevented any planetary bodies from
forming in the gap between Mars and Jupiter,
causing the small objects that were there to collide
with each other and fragment into the asteroids seen
today.
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Asteroids can reach as large as Ceres, which is 940 km (about 583 miles) across. On the other
hand, one of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6
meters) across.
Nearly all asteroids are irregularly shaped, although a few are nearly spherical, such as Ceres.
They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460
km) in diameter. The surfaces of most asteroids are thought to be covered in dust.
As asteroids revolve around the sun in elliptical orbits, they rotate, sometimes tumbling quite
erratically. More than 150 asteroids are also known to have a small companion moon, with
some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly
equal size orbit each other, and triple asteroid systems are known as well. Many asteroids
seemingly have been captured by a planet's gravity and become moons — likely candidates
include among Mars' moons Phobos and Deimos and most of the distant outer moons of Jupiter,
Saturn, Uranus and Neptune.
The average temperature of the surface of a typical asteroid is minus 100 degrees F (minus 73
degrees C). Asteroids have stayed mostly unchanged for billions of years — as such, research
into them could reveal a great deal about the early solar system.
Asteroids come in a variety of shapes and sizes. Some are solid bodies, while others are
smaller piles of rubble bound together by gravity. One, which orbits the sun between Neptune
and Uranus, comes with its own set of rings. Another has not one but six tails.
- See more at: http://www.space.com/51-asteroids-formation-discovery-andexploration.html#sthash.MeUo1exJ.dpuf
ASTEROIDS
1. (b) State the position of the asteroid belt in the
solar system.
situated between (orbits of) Mars and Jupiter;
PROPERTIES OF A STAR
2. The peak in the radiation spectrum of a star X is at
a wavelength of 300nm. Show that the surface
temperature of star X is about 10000K.
l = 2.9 x 10-3 / T
T = 2.9 x 10-3 / 300 x 10-9
T = 9666.67 K
T ≈ 10000 K
(b) The radius of star X is 4.5RS where RS is the radius of
the Sun. The surface temperature of the Sun is 5.7×103
K.
Determine the ratio
Use the L = sr2T4
luminosity of star X
luminosity of the Sun
Lx srx T 4 rx T 4 4.5 Rs  9700 K 
 2 4  2 4 
2
4
Ls srs T
rx T
Rs 5700 K 
2
2
Lx
 169.8  170
Ls
2
4
(c) On the Hertzsprung–Russell diagram, label the
position of star X with the letter X.
X
VARIABLE STARS AND SUPERNOVAE
Cepheid variable stars are used as “standard
candles” by astronomers.
(a) (i) State what is meant by a standard candle.
object of known luminosity/power
IB Physics SL revision - Option E (Astrophysics) 4
https://www.youtube.com/watch?v=KNqn5HUP5ME
• CEPHEIDS AS 'STANDARD CANDLES'
• When we observe another galaxy, we can assume that all its stars are around the
same distance from the Earth. A source of known luminosity in that galaxy enables
us to make comparisons with all the other stars in the galaxy to determine their
luminosity. Cepheid variable stars, which are thousands of times more luminous
than the Sun, provide us with such a benchmark, known in astronomy as a
"standard candle". By observing the period of any Cepheid, you can deduce its
absolute brightness. Then, using an observation of its apparent brightness, the
distance to it can be calculated. Henrietta Leavitt first discovered the periodluminosity relationship of Cepheids in 1912 for Cepheids in the nearby galaxy called
the Small Magellanic Cloud.
• DISTANCE TO CEPHEIDS
• It is possible to estimate the distance to a Cepheid in a far-off galaxy as follows:
firstly, locate the Cepheid variable in the galaxy, then measure the variation in its
brightness over a given period of time. From this you can calculate its period of
variability. You can then use the luminosity-period graph (below) to estimate the
average luminosity. Finally, armed with the average luminosity, the average
brightness and using the inverse square law, you can estimate the distance to the
star.
(ii) Outline the properties of a Cepheid star that allow
it to be used as a standard candle.
(ii) luminosity varies with time in a regular way;
(average) luminosity related to period of
variation; high luminosity so visible from great
distances;
(iii) Explain how astronomers use their observations of a
Cepheid star to determine the distance from the star to
Earth.
(iii) the period of the variation of
luminosity/apparent brightness/apparent
magnitude is measured; the luminosity/absolute
magnitude is determined from period; apparent
magnitude/brightness is measured (on Earth);
d 
m  M  5 log 
 10 
is used to compute d;
L
b
4d 2
(b) Astronomers can also determine distance by
observing supernovae. A supernova was observed
from Earth. At the peak of its emissions, the supernova
had an absolute magnitude of –20 and an apparent
magnitude of 13. Determine, in parsec, the distance
from Earth to the supernova.
d 
m  M  5 log  
 10 
d 
13  (20)  5 log  
 10 
d 
33  5 log  
 10 
d 
6.6  log  
 10 
d
106.6 
10
40Mpc
1 pc = 3.26 LY
1 parsec = 3.26 light years
https://www.youtube.com/watch?v=jr3MWGhYXTg
• This question is about Newton’s model of the
universe.
• Newton suggested that the universe is infinite,
uniform and static.
• For each of Newton’s three suggestions, outline one
piece of current astronomical evidence that
contradicts the suggestion.
• infinite: cosmic microwave background is observed
consistent with cooling from a finite beginning / use of
the Hubble constant to find universe age / bright
universe not
• observed whereas an infinite universe would be
completely bright;
• uniform: significant empty distances visible between
galaxies / reference to galactic
• clusters/super clusters / the greater the observed
distance of galaxies, the greater the
• red-shift;
• static: red-shift indicates expansion of universe / galaxies
observed to be moving
• relative to Earth;