what does it mean when an object in space is as faint as 31st magnitude?

Question

the hubble space telescope can detect objects as faint as 31 magnitude

Answer 1

Magnitude is a measurement of the brightness of an object.

The sun is -26.7 magnitude. The star Vega is about 0 magnitude. The dimmest stars your eye can see in a perfectly dark place on a clear, moonless night are about 6.5 magnitude.

A difference of 6 magnitudes corresponds to a 100-fold difference in illumination. In other words, a 0-magnitude star is 100 times as bright as a 6th-magnitude star.

A 31st-magnitude star is 31 magnitudes dimmer than Vega, in other words a tiny fraction as bright.

Answer 2

A star of magnitude m is 100 times brighter than a star of magnitude m+5.

The brightest stars in the sky are mostly around magnitude 0 or 1, and the faintest star you can see in a dark sky is about magnitude 6. Thus, a star of magnitude 31 is 10000000000 (ten billion) times fainter than the faintest star you can see naked-eye.

There are several reasons why the Hubble telescope can detect such faint objects:
1) Its mirror is 94 inches in diameter, so it collects a lot more light than your eye does.
2) It's in space, so the light from a star is not blurred by the atmosphere.
3) It uses very sensitive detectors to pick up the light.
4) Unlike your eye, a telescope can watch an object for a long time and keep accumulating more light in the image.

Answer 3

The faintest we can see with the naked eye is 6th magnitude (average person at a dark site -- your milleage may vary).

The magnitude scale works in reverse (bigger number = fainter object).
The scale is geometric (each step is 2.514 times brighter than the next one -- or fainter, depending which way you're going).

A difference of 5 magnitudes corresponds to 100 times.
A difference of 10 corresponds to 100*100 = 10,000 times
A difference of 15: 1,000,000
20: 100,000,000
25: 10,000,000,000

Magnitude 31?

31-6 = 25

The Hubble telescope collects light 10,000,000,000 times more efficiently than the human eye. Therefore it can see much fainter objects.

It does so in many ways: it has a much larger "aperture" (2,400 mm instead of 7 mm), it does not have to look through the atmosphere, it can integrate light over a period of time (allowing photons to accumulate until the detectors... detect it -- the human eye's integration time is of the order of 1/10 to 1/20 of a second).

Answer 4

If we are talking about optical wavelength, that means HST is so sensitive that it can detect an object that is 10^23 times fainter than the sun. The detection limit varies with wavelength