Transcript Option E - IB Physics Ancaster

Astrophysics
Bits
• The solar system is the collection of bodies gravitationally
bound to the sun.
• The 8 planets orbit the sun in elliptical orbits and are the
major bodies of the solar system.
• In order of increasing size: Mercury, Mars, Venus, Earth,
Uranus, Neptune, Saturn and Jupiter.
• In order from the sun: Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune.
• Moons orbit planets and are called natural satellites.
• Between Mars /Jupiter is the asteroid belt/dwarf planets.
• Comets are a mix of ice, dust and gas and orbit in
parabolic orbits (mostly).
Star Groups
• A galaxy is a collection of 100 billion stars, dust
and gas held together by gravity.
• The shapes of galaxies: spiral, elliptical or
irregular. (Our galaxy is spiral.)
Galaxies
• A group of stars in a a galaxy closely bound is called a
cluster.
• A globular cluster has 10 5 stars arranged with some
symmetry and densely packed in the centre.
• An open cluster or moving cluster is irregular and has
hundreds of stars.
• Stars grouped together (at varying distances from Earth)
that form patterns are called constellations.
• The stars appear to move, but really it’s the Earth’s
rotation that gives this appearance.
Distances
• A light year is the distance travelled by light in one year
= 9.46 x 1015 m. (1ly = 3x108x365x24x60x60 m)
Examples:
1) How long does it take light to travel to us from the
moon if it is 370 000 km away?
Ans: 1.2 s
2) How far away is the sun in light-minutes if it is 150 000
000 km away?
Ans: 8.3 min
Distances
Examples:
3) Proxima Centauri is 4.3 light-years away, and the
fastest man-made object moves at 150 000 ms -1. How
long would it take to travel to PC?
Ans: 8600 years!
4) The most distant visible star (to the eye) is Epsilon
Aurigi at 5000 ly. How far is this in m?
Ans: 4.73 x 1018 m
5) The most distant galaxy visible to the eye is
Andromeda M31 at 2.2 x 106 ly. How far is this in m?
Ans: 2.1 x 1022 m
Stellar Radiation
• Nuclear fusion is the energy source for stars.
• 2 H-atoms collide at speeds to fuse to He.
• The energy is released as radiation.
• The pull of gravity is balanced by the outward
pressure from fusion.
Luminosity and Apparent Brightness
• Luminosity, L, is the total power output of a star
in Watts.
• The sun’s luminosity is Lsun = 3.9 x 1026 W
• The apparent brightness (b) of a star is the power
output per unit area at a certain distance, d,
measured in Wm-2.
𝐿
𝑏=
4𝜋𝑑 2
Ex: If you were 3 m away from a 100W bulb, then
b = 100/[4x3.14x32] = 0.9 Wm-2.
Examples
1) What is the apparent brightness of the sun (Lsun
= 3.9 x 1026 W) at Earth (1.5 x 10 11 m away)?
Ans: 1.4 x 102 Wm-2
2) What is the apparent brightness of the sun one
light-year away?
Ans: 3.5 x 107 Wm-2
Stellar Distances
Parallax Method: Hold up one finger at arm’s length
and observe with each eye – it appears to move!
• The Earth is used in 2 positions 6 months apart and a
nearby star is observed.
• The star’s position relative to the distant stars
appears to change.
• The angle of parallax, p, is the difference in angular
positions as seen from the Earth and the sun, in
seconds of arc.
The closer the star: the larger the parallax.
Parallax Diagram
p
d
Parallax Example
Alpha Centauri has a parallax of 0.760 arcseconds.
Note: One degree is 60 minutes, one minute is 60
seconds, so one arcsecond is 1/3600 of a degree.
Tan p = O/A so A = O/tanp
d = 1 AU/tan0.76 arc sec
= 271 000 AU
Convert this distance to m and ly.
Ans: 4.1 x 1016 m = 4.3 ly
Parsecs
• A parsec is the distance where a star has p = 1
arcsecond.
• The distance, d, in parsecs (pc) is d = 1/p when
p is given in arcseconds.
• Ex: How far away is Alpha Centauri in pc?
d = 1/p
d = 1/0.760 = 1.3 pc
Parsec examples
1) What is the distance in pc when p = 0.010 arcsec?
0.002 arcsec?
Ans: 100 pc, 500 pc
2) If the error in parallax is +/- 0.001 arcsec, what errors
are associated with the answers from 1?
Ans: +/- 10 pc, +/- 200 pc
∴The error is quite large for stars farther away!