Astronomy 202 Stars, Galaxies, and the Universe Dr. Jeff Kriessler

What is science?

Discussion Science is a collection of facts

Discussion Scientists are looking for the truth

My view Science is a collection of explanations that are useful in scientists attempts to predict the future.

Newton’s law of gravity

It is inconceivable that inanimate brute matter should, without mediation of something else which is not matter, operate on and affect other matter without mutual contact.... That gravity should be innate, inherent and essential to matter, so that one body may act upon another at-a-distance, through a vacuum, without the mediation of anything else by and through which their action may be conveyed from one to another, is to me so great an absurdity that I believe no man, who has in philosophical matters a competent faculty of thinking, can ever fall into it. (Newton. Letter to Richard Bentley 25 Feb. 1693)

Discussion Scientists always need to keep an open mind

Models in science A scientific model needs to: 1)Explain past observations 2)Predict accurately future observations 3)Should be the simplest possible explanation

Occam’s Razor If two or more models encompass the observations equally well, the simplest model is preferred.

The Night Sky

Discussion Even though there are 6000 stars that are bright enough to see without a telescope, we can only ever see about 3000 at any given clear night. Why is that?

Discussion Why can’t we see stars during the day?

Light Pollution

Stellar magnitudes We specify the apparent brightness of stars by the magnitude scale. Brighter stars have a smaller magnitude while fainter stars have a larger magnitude. The brightest star, Sirius has a magnitude –1.4 The faintest stars visible to the unaided eye have a magnitude of about 6.

Discussion The magnitude system that astronomers use is based on the way our eyes scale the intensity of light, which is not linear but logarithmic. What do you think would be the advantage of having eyes which scale intensity in such a fashion?

The celestial sphere We can picture the sky as the inside of a hemisphere.

Discussion On Earth’s surface, also a sphere, we specify positions using latitude and longitude. How does this system work?

Positions in the sky Any position in the sky can be specified by two angles, just like latitude and longitude on the Earth’s surface.

Some terminology Zenith – the point in the sky directly over head. Meridian – an arc drawn from due north on the horizon to the point due south on the horizon which passes through the zenith.

Diurnal (daily) motion of the stars Like the Sun, the stars generally rise in the east and set in the west. This daily motion of the stars is caused by the rotation of the Earth.

All the stars appear to circle a point in the sky called the celestial pole. In the north this point lies near the star Polaris, the north star. In the south this point lies near the Southern Cross.

Circumpolar stars Not all stars rise and set. Some stars, called circumpolar, always appear above the horizon.

Star trails on the celestial equator

The celestial poles and equator north celestial pole – point in the sky directly above the Earth’s north pole south celestial pole – point in sky directly above the Earth’s south pole. celestial equator – midway between the celestial poles (90 degrees away), lies directly above the Earth’s equator.

Discussion This there any place on Earth where all the visible stars are circumpolar, never falling below the horizon? Explain.

Discussion Is there any place on Earth where none of the visible stars are circumpolar? Explain.

Discussion If we were at the North pole, how far above the horizon would the North celestial pole be?

Discussion How high above the north horizon is the north celestial pole from Earth’s equator?

Discussion How high above the northern horizon would the north celestial pole appear in Cleveland which has a latitude of about 42 degrees?

Discussion The biggest problem in navigating using the stars was in determining one’s longitude. Explain why this is so difficult?

Celestial Coordinates Declination (Dec) – measured in degrees minutes and seconds, from the celestial equator. Right Ascension (RA) – measured in hour minutes and seconds from the vernal equinox.

Annual motion of the stars The same stars are not visible all year long. Any given non-circumpolar star will set 4 minutes early each day until it becomes lost in the glare of the setting Sun. This motion is caused by Earth’s yearly motion around the Sun.

In other words While the Sun takes 24 hours to traverse the sky, the stars take only 23 hours and 56 minutes. The Sun moves 4 minutes eastward each day relative to the stars. Thus different stars are visible at different times of the year.

Solar and sidereal days Solar day – 24 hours Sidereal day – 23 hours 56 minutes is the actual rotation period of the Earth

The ecliptic The ecliptic is the annual path through the sky that the Sun appears to take. In other words, the ecliptic is the plane of Earth’s orbit projected onto the stars.