astronomy 100 the solar system tuesday, wednesday, thursday tom burbine [email protected]
TRANSCRIPT
What is a galaxy?
HW #2
• Due tomorrow (June 10th)
What is a galaxy?• Is a massive, gravitationally bound system
consisting of stars, gas and dust, and dark matter. Galaxies can contain between ten million and a trillion stars
• Dark matter is matter that does not emit or reflect enough radiation to be seen, but whose gravitation effects can be felt
http://en.wikipedia.org/wiki/Image:NGC_4414_%28NASA-med%29.jpg
When we are looking at stars or galaxies
• We are looking into the past
Light-year is the distance light travels in a year.
Milky Way Galaxy
• Milky Way is 100,000 light years in diameter• There are ~200 billion stars in the Milky Way (estimates
from 100-400 billions stars)
http://www.venusproject.com/ecs/images/photos/galaxy.jpg
What is the Universe?
What is the Universe?• Sum total of all matter and energy – all galaxies
and everything between them
• Observable universe – portion of the universe that can be seen from Earth, probably only tiny portion of the whole universe
~93 billion Light-years
wide
What causes seasons?
• The tilt of the Earth’s axis relative to the ecliptic
Seasons
Solstices
• Summer Solstice –June 21 – Northern Hemisphere receives its most direct sunlight
• Winter Solstice – December 21 – Northern Hemisphere receives its least direct sunlight
Equinoxes
• Sun shines equally on both hemispheres
• Spring Equinox – March 21 – Northern Hemisphere goes from slightly tipped away from the Sun to slightly tipped towards
• Fall Equinox – September 21 - Northern Hemisphere goes from slightly tipped toward from the Sun to slightly tipped away
Why does the orbital difference not matter?
Reasons
• There is only a 3% difference in the distance from the Earth to the Sun at its farthest and closest point
• The Earth is actually closer to the Sun in the winter than in the summer
Mars is now visible in the sky
• http://www.post-gazette.com/pg/10025/1030378-369.stm
Angular size
• We measure distances in the sky using angles
• 180o in the observable sky
More precise distances
• 1 degree = 60 arcminutes (symbol ´)
• 1 arcminute = 60 arcseconds (symbol ´´)
• So something that is 2 degrees, 10 arcminutes, 22 arcseconds would be written as
• 2o 10´ 22´´
Terminology for looking at the sky
Celestial Sphere
• an imaginary sphere of infinite extent on which all celestial objects appear to lie
• http://www.skyandtelescope.com/s?action=login
Celestial Sphere
What is a constellation?
Constellations
• People refer to constellations as a pattern of stars
• Astronomers refer to constellations as specific regions of the sky
• In 1928, the IAU (International Astronomical Union) decided there were 88 constellations
• Many of the constellation names go back thousands of years
Constellations
• The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!).
• The real purpose for the constellations is to help us tell which stars are which, nothing more.
What is thisconstellation?
Orion
Bigger the star, the brighter it is
Orion was the son of the god of the sea, Poseidon and a great hunter.One story is that he made an enemy of Hera who sent a scorpion to sting him.
Orion was restored to health by Ophiuchus, the first doctor of medicine. Another story is that Artemis was tricked by by Apollo to shoot an arrow at Orion. When he died, Poseidon asked Zeus to put him among the stars.
Ursa Major
• Ursa Major, the Great Bear, was identified with a bear by native American Indians of the Northeastern United States and the ancient Greeks.
• The name common in Britain, the Plough,seems to have a medieval origin,
• Another common name among northern European cultures is the Wain, a shortened form of wagon
What are the constellations named after• 14 men and women• 9 birds • 2 insects• 19 land animals• 10 water creatures• 2 centaurs• one head of hair • a serpent• a dragon• a flying horse• a river• 29 inanimate objects
• Originally considered part of Leo’s tail
• Named after Queen Berenice II of Egypt, wife of Ptolemy III Euergetes (246 BC - 221 BC)
• Around 243 BC, the king undertook a dangerous expedition against the Syrians, who had murdered his sister.
• Berenice swore to the goddess Aphrodite to sacrifice her famous long hair if her husband returned safely.
• He did, she had her hair cut, and placed it in the goddess' temple.
• By the next morning, the hair had disappeared. • To appease the furious king and queen (and save the lives of
the temple priests), the court astronomer, Conon, announced that the offering had so pleased the goddess that she had placed it in the sky.
• He indicated a cluster of stars that at the time were identified as Leo's tail, but now have been called Berenice's Hair.
Zodiac
• The zodiac is an imaginary belt in the heavens extending approximately 8 degrees on either side of the Sun's apparent path (the ecliptic), that includes the apparent paths of the Moon and the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune.
Question:
• Why do all the planets seem to follow the same path?
Answer:
• The planets, the Earth, and the Sun all tend to fall in the same plane called the ecliptic
Why don’t all the constellations have ancient names?
• Ancient cultures such as the Greeks and Egyptians could not see the constellations in the Southern Hemisphere
Question:
• Why is the path of the constellations on the zodiac not on the celestial equator?
Answer:• The rotation axis of the Earth is
inclined with respect to the ecliptic
• Polaris is called the North Star
• Brightest star in the constellation Ursa Minor.
• 48th brightest star in the night sky
• It is very close to the north celestial pole, making it the current northern pole star.
• Polaris' altitude, or height above the horizon, is equal to an observer's latitude.
Long Term Changes
Precession
• Earth precesses like a top
• Precession - phenomenon by which the axis of a spinning object (e.g. a part of a gyroscope) "wobbles" when a torque is applied to it
Forces
• For a top, the force is gravity, which is trying to pull the top down
• For the Earth, the forces are due to the pull of the Sun and Moon, which is trying to align the Earth’s axis with the ecliptic
Because of precession
• The position of a star that corresponds to the North Celestial Pole changes
Models
• When you have a model of how something works, you should be able to predict what will happen
• If observations do not fit the model, either the observations or the model is wrong
• The ancient astronomers wanted to predict the positions of planets in the sky
What did the ancients think
• That the Earth was the center of the universe
• That the celestial sphere was rotating around the Earth
• However, there was two observations that caused problems with this idea– Apparent retrograde motion– Inability to detect stellar parallax
Greek model
Apparent Retrograde Motion = “backward” motion
Retrograde Motion
Retrograde
• Planet appears to go backwards in its orbit
Stellar Parallax
• Stellar Parallax – The apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in the Earth’s orbit of the Sun each year
The distance the star moves is greatly exaggerated in this figure.Stellar parallax can only be seen by a telescope.
Ancient astronomers could not detect stellar parallax
• If Earth orbited the Sun, ancient astronomers believed that they would see differences in angular separation of stars as the Earth rotated around the Sun
• Since they saw no changes in angular separation of the stars, they assumed the Earth was the center of the universe
• They could not fathom that stars are so far away that stellar parallax is undetectable by the human eye
Ptolemy’s (100-170 AD) Model of the Universe
Nicholas Copernicus (1473-1543)
• Copernicus came up with a model that the Earth revolves around the Sun
• Similar to what Aristarchus (310 – 230 BC) thought 2000 years before
• However, Copernicus’ models did not match observations since he wanted everything to arouind in perfect circles
Tycho Brahe (1546-1601)
• Tycho Brahe was the greatest naked eye observer of all time
• He lived before the invention of the telescope• His observations of the alignment of Jupiter and
Saturn occurred two days later than when predicted by Copernicus
• Tycho came up with a model where the planets orbit the Sun but the Sun orbits Earth
Johannes Kepler (1571-1630)
• Tried to match circular orbits to Tycho’s data
• Couldn’t do it
• Because Tycho’s observations were so good, Kepler had to come up with a new model
Kepler was trying to match an orbit to Tycho’s observations of Mars
• “If I believed that we could ignore these eight minutes of arc, I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those 8 minutes pointed to the road to a complete reformation in astronomy.”
• Kepler came up with his 3 laws of planetary motion
Kepler’s 1st Law
• The orbit of each planet about the Sun is an ellipse with the Sun at one focus (there is nothing at the other focus)
Differences between ellipses and circles
Eccentricity (e)
• e = distance between the two foci/length of major axis
• e of circle is 0
• The larger e becomes, the more eccentric the orbit
Definitions
• Perihelion – planet closest to the Sun
• Aphelion – planet farthest from the sun
• Semi-major axis (a) – the average of a planet’s perihelion and aphelion distances
Kepler’s 2nd law
• As a planet moves around its orbit, it sweeps out equal areas in equal times.
• This means that the planet travels faster when it is nearer the Sun and slower when it is farther from the Sun
Kepler’s 3rd Law
• More distant planets orbit the Sun at slower average speeds, obeying the precise mathematical relationship
p2 = a3
where p is a planet’s orbital period in years and a is the average distance from the Sun in
astronomical units.
Calculations
• The period for the Earth to go around the Sun is 1 year
• The average distance of the Earth to the Sun is 1 Astronomical Unit
How long does it take Jupiter to go around the Sun
• If Jupiter is 5.2 Astronomical Units from the Sun, how long does it take Jupiter to go orbit the Sun once
• p2 = a3 = 5.23 = 140.6
• p = √140.6 = 11.9 years
Another example
• Mercury is 0.4 Astronomical Units from the Sun.
• How long does it take Mercury to orbit the sun once?– A) 1 year– B) 3 months– C) 9 months– D) 5 years
The calculation
• p2 = a3 = 0.43 = 0.064
• p = √0.064 = 0.25 years
• An asteroid takes 8 years to go around the Sun
• How far is the asteroid away from the Sun?– A) 1 AU– B) 3 AU– C) 4 AU– D) 8 AU
The calculation
• a3 = p2 = 82 = 64
• a = (64)1/3 = 4 AU
You can calculate a planet’s orbital speed
• Since you know a planet’s orbital distance
• And you know its orbital time
• You can calculate a planet’s average orbital speed
Laboratory
• http://www.hulu.com/watch/63319/cosmos-the-harmony-of-the-worlds#x-0,vepisode,1,0
Any Questions?