Astronomy 202

Stars, Galaxies, and the Universe

Dr. Jeff Kriessler

What is science?

Discussion

Science is a collection of facts

Discussion

Scientists are discovering the truth

My view

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

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MMGF

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.