What do the stars tell us?

60 downloads 7109 Views 7MB Size Report
Seven Wanderers. • Among the fixed stars, seven wanderers (planets) were known in ancient times: The Sun, the Moon,. Mercury, Venus, Mars, Jupiter and.

What do the stars tell us? Astronomy, Astrology, and Cosmology - a very quick tour of our universe

Power and Magic in the Sky You can keep precise track of the seasons by observing the apparent motion of the Sun and the rising times of bright stars. In early agrarian societies, such knowledge was critical for survival.

Power and Magic in the Sky • Sunrise and sunset positions on the horizon mark the seasons.

Power and Magic in the Sky • The first time a bright star (such as Sirius) can be seen rising ahead of the Sun is another seasonal marker

Seven Wanderers • Among the fixed stars, seven wanderers (planets) were known in ancient times: The Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn.

The Origin of Astrology • The Sun’s apparent motion carries vital information about the calendar. • The Moon’s variations are also useful. • Maybe the other wandering bodies (planets) are trying to tell us something, too? • From: Events affecting all, to predictions concerning royalty, to predictions for any individual.

Now we know a lot more about the planets (Including what is a planet*)

and we know that they cannot influence individual events and people on Earth. * and there are eight, not counting Sun and Moon

How many planets? or, why is Pluto no longer a Planet? • 1930: Pluto was thought to be perturbing Neptune. It isn’t. • 1978: Pluto has a moon; this means Pluto is even smaller. • 2005: There is a body out there that is bigger than Pluto (now called Eris). • 2006: Either Pluto is a planet, and so is Eris, and so is Ceres, and perhaps so, also, is Pluto’s moon Charon and a number of other not-yet-discovered bodies …

Or Pluto and Eris are dwarf planets and there are five kinds of objects in the solar system: • Terrestrial (Earth-like) planets Mercury, Venus, Earth and Mars - rock & metal • Jovian (Jupiter-like) planets Jupiter, Saturn, Uranus and Neptune - mostly gas • Dwarf planets: Big enough to be round (shaped by gravity) and not orbiting other planets. • Moons: Some are bigger than Mercury. • Small solar system bodies: Not big enough to be round.

The Stars - Key to Past and Future Today, the stars provide us with information on the more distant past and future of Earth, the Sun, the solar system and our universe. Nearly all the light collected by telescopes comes from stars.

Stars tell us how old galaxies are. • Starlight comes from nuclear reactions and/or gravity. • We can determine the lifetime of a star from its fuel supply and how fast it is using it. • Stars with a lot of fuel spend it much faster, so they have short “lifetimes”. • Stars like the Sun have fuel for 10 billion years of stable consumption.

The ages of galaxies and star clusters A young cluster has bright blue stars

Luminosity (Power being radiated)

Hot, blue ……….cool, red

An old cluster has red giants but no bright blue stars

The age of the universe as a whole: When everything was tightly packed together

z

NOW

THEN

A big problem in 1990 If the galaxies always moved away at the same speed, then by measuring distance now and speed now we can deduce when they left: Age = distance / speed Measurements gave distance / speed between 10 and 15 billion years 10 Gyr is younger than the oldest stars!!!

A big problem in 1990 If the galaxies always moved away at the same speed, then by measuring distance now and speed now we can deduce when they left: Age = distance / speed We expected that galaxies were slowing down (from gravity) which would make this problem worse.

Then came a surprise The hard thing to measure is distance. You need a good “standard candle” that is also very bright (so you can see it far away). Supernovae are very bright, but they are not all alike. However, one type, SN Ia, appear to follow a rule relating how bright they are (as standard candles) to how fast they fade.

Above: Different decline rates, different peak brightness Below: corrected to a single standard candle

SN Ia These eject material that has no hydrogen and therefore the star that exploded must be hydrogen-free; the most common hydrogen-free stars are white dwarf stars made of He, of C and O, or of Mg and Ne. A white dwarf has a mass about like that of the Sun crammed into a space the size of Earth - a volume a million times smaller.

Inside a white dwarf - SN Ia Electrons are so close together that they run into a limit: no more than two electrons per box in six dimensions. The six dimensions are position (3D) and speed (another 3D). When the low speed boxes are full, added electrons have to be moving at high speed. When the mass is ~ 1.4 solar masses, some of the electrons are moving at close to the speed of light. These start nuclear reactions that detonate the supernova explosion.

SN Ia To reach 1.4 solar masses, the white dwarf must be accreting mass. To explode with no hydrogen spectrum, it must be accreting hydrogen-free material. The most likely source of hydrogen-free material is another white dwarf. Thus SN Ia are all explosions of hydrogen-free white dwarf stars that grow to 1.4 solar masses - this makes the explosions nearly identical.

Then came a surprise Using these supernovae, distances were measured very carefully, by two groups working independently to check each other. They found that the expansion is speeding up - accelerating - instead of slowing down.

We were all sure they were wrong … ..but no one could find an error, and both groups got the same answer.

Accelerating ?!?! I didn’t believe it until I saw that it solved several problems at once: If the expansion is accelerating, then it was slower, and the Universe is older comfortably older than the oldest star.

The best mathematical theory of the original “Big Bang”, inflation theory, predicted that space should be very flat, and with this accelerating expansion, it is very flat.

Acceleration and the shape of the universe The curvature of the universe is determined by ! = !matter + ! darkmatter +! other Regular matter (stars, planets, dust, gas.. all we can see) !matter = 0.04. Dark matter (matter we know is there only because of its gravity) !darkmatter = 0.24 We had

Suggest Documents