star brightness?

Question

When you look at the night sky, some stars look bright and others less bright. How do astronomers measure the brightness of a star – what measurement do they use? What is the brightest object in the night sky? (not on any specific night, just in general)

Answer 1

THE EVOLUTION OF THE MAGNITUDE SYSTEM FROM HIPPARCHUS TO POGSON



A scale of magnitude (bright stars are first magnitude, and dim ones are sixth magnitude) was developed by the Greek astronomer Hipparchus, popularised by Ptolemy in his Almagest and refined in Victorian times by Pogson.



Wikipedia says of the Greek system:



6th magnitude stars were at "the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered to be twice the brightness of the following grade (a logarithmic scale). This somewhat crude method of indicating the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus.



This original system did not measure the magnitude of the Sun. (The Sun, according to Prolemy's model of the universe was a planet rotating around the earth, and not a star at all).



In 1856, Pogson formalized the system by defining a typical first magnitude star as a star that is 100 times as bright as a typical sixth magnitude star; thus, a first magnitude star is about 2.512 times as bright as a second magnitude star. The fifth root of 100, an irrational number (about 2.512) is known as Pogson's Ratio.



Pogson's scale was originally fixed by assigning Polaris a magnitude of 2. Astronomers later discovered that Polaris is slightly variable, so they first switched to Vega as the standard reference star,



The modern system is no longer limited to 6 magnitudes or only to visible light. Very bright objects have negative magnitudes. For example, Sirius, the brightest star of the celestial sphere, has an apparent magnitude of −1.46. The modern scale includes the Moon and the Sun; the full Moon has an apparent magnitude of −12.6 and the Sun has an apparent magnitude of −26.73. The Hubble Space Telescope has located stars with magnitudes of 30 at visible wavelengths and the Keck telescopes have located similarly faint stars in the infrared."



APPARENT MAGNITUDES AND ABSOLUTE MAGNITUDES



An apparent magnitude is how bright a star looks to an observer from earth. The Sun looks as bright as it does, because it is only 8 light minutes away from us. Whereas the nearest star is over 4 light years away. Astronomers decided to develop a scale of absolute magnitudes, that expressed how bright a star would look if it were 10 parsecs away (about 32.6 light years away), so that they could compare like wlth like.



As your question implies, as between two stars one quite bright but a long way away and one not so bright but very near, which is really the brighter of the two if we can ignore the distances involved? i.e. which has the greater luminosity? The scale of Absolute Magnitudes is the way that that comparison can be made.



For objects within our Galaxy with a given absolute magnitude, 5 is added to the apparent magnitude for every tenfold increase in the distance to the object i.e. it is one hundredth as bright if it is ten times the distance away (an inverse square law applies). This relationship does not apply for objects at very great distances (far beyond our galaxy), since a correction for General Relativity must then be taken into account due to the non-Euclidean nature of space.



The lower an object's absolute magnitude, the higher its luminosity.



Many stars visible to the naked eye have an absolute magnitude which is capable of casting shadows from a distance of 10 parsecs; Rigel (-7.0), Deneb (-7.2), and Betelgeuse (-5.6).



For comparison, Sirius has an absolute magnitude of 1.4 and the Sun has an absolute visual magnitude of 4.83, i.e. if the Sun were 10 parsecs away it would be barely visible to the naked eye on all but the clearest nights. Some people find this surprising as they are so used to the Sun as a very bright and important object in our lives. But on a cosmic scale it is really quite ordinary. 4.83 is dimmer than Ganymede's apparent magnitude.



Absolute magnitudes for stars generally range from -10 to +17. The absolute magnitude for galaxies can be much lower (brighter). For example, the giant elliptical galaxy M87 has an absolute magnitude of -22.



THE BIG PICTURE



Apparent Magnitude. Celestial Object

−26.73 Sun

−12.6 full Moon

−4.7 Maximum brightness of Venus

−2.9 Maximum brightness of Mars

−2.8 Maximum brightness of Jupiter

−1.9 Maximum brightness of Mercury

−1.5 Brightest star (except for the sun) at visible wavelengths: Sirius

−0.7 Second brightest star: Canopus

0.7 Maximum brightness of Saturn

3 Faintest stars visible in an urban neighbourhood with naked eye

4.6 Maximum brightness of Ganymede

5.5 Maximum brightness of Uranus

6 Faintest stars observable with naked eye

7.7 Maximum brightness of Neptune

12.6 Brightest quasar

13 Maximum brightness of Pluto

27 Faintest objects observable in visible light with 8m ground-based telescopes

30 Faintest objects observable in visible light with Hubble Space Telescope

Answer 2

Astronomers use a scale called 'magnitudes' to measure brightness. The relative magnitude of a star is how bright it appears in the sky. The absolute magnitude is how bright the star would appear if it were 10 parsecs (about 31 light years) away. The magnitude scale is linked with the amount of energy recieved by the star in a certain period of time (luminosity) and depends on the color range measured.



The brightest object in the sky is the sun (of course). Next is the moon. The planets (venus and Jupiter in particular) can be the next brightest. The star Sirius is the brightest night-time star.

Answer 3

A star's actual brightnes (luminosity) depends on it's temperature. Several things can affect how bright it looks to us. If it is a cooler star it is inherently dimmer than a hotter star at the same distance. Stars that are further away are also dimmer than closer stars. This is due to distance and to the fact that there is dust in between the stars. The more dust there is the dimmer the star will seem.

The luminosity of a star is a property of the star, but the magnitude of a star is what we measure on Earth. You use a photometer to measure the apparent magnitude of the star. Smaller magnitudes mean brighter stars. Luminosity can be calculated from the magnitude measurements if you know the distance and spectral type, using the distance modulus.

The brightest object in the night sky is the moon. The brightest star is Sirius in the constellation Canis Major.

Answer 4

Magnitude measures star brightness. The brightest object in the night sky would be the moon.

Answer 5

Brightness of celestial objects is measured in magnitudes. Magnitudes is a logarithmic scale, meaning a star of magnitude 1 is 2.5 times brighter than a star of magnitude 2, which in turn is 2.5 times brighter than magnitude 3.



The brightest fixed star (that is, non-planet) is Sirius, at magnitude -1.5. The brightest planet is Venus, which can reach magnitude -4.3.

Answer 6

Apparent magnitude is the apparent brightness of a celestial body as seen from earth. The brighter the body is the lower its apparent magnitude, this can even be negative. Our sun (the brightest object in the sky) has an apparent magnitude of -26.73, while the moon (the brightest object in the night sky) has an apparent magnatude of -12.6.

Answer 7

There is a reason for it. It is because the way humans rate, like 'First Class' (1st Class) and 'Second Class' (2nd Class) is always lower than 'first'. But if you have to rate something better than first, you may have to (1 - n)th class that'd be negative the higher 'n' is. Earlier, men (earlier men were always avid astronomers in the absence of lamps/lights or night life) made a count of 20 brightest stars and called them the 'First Magnitude' stars. Then another class of (about 50) dimmer stars including our Pole Star ('Polaris') and called them the 'Second Magnitude' stars or 'Stars of Magnitude 2'. Then the scientific methods started and Photometry was applied to star brightness. It became possible to give the brightness as 'Magnitude 1.33' (Alpha Crucis; the 20th brightest). The 5th brightest star, 'Vega' has the 'Magnitude 0.03' accordingly. That leaves the even brighter 4 beyond 0.00 in the negative region. These are : Sirius (-1.46), Canopus (-0.72), Arcturus (-0.04), Rigil Kentaurus or Alpha Centauri (-0.01). These are the only 4 being the brightest, with negative Magn. numbers. All this, is after Pogson proposals. In 1856, NR Pogson proposed a ratio of 100 for a difference in magnitude of 5. So on it went till the barely visible stars were assigned Magnitude 6. 15th brightest 'Spica' with 'Magn.0.98' is 100 times brighter than 'Lambda (19) Cancri', '87 Piscium', '39 Andromedae', '7 Persei', '69 Pegasi' and another 40 more with 'Magn.5.98'; that is higher by 5 magnitudes. It implies that each magnitude difference in negative direction corresponds to a brightness ratio of 2.511886 times. Spica (Magn.0.98) is brighter by that ratio over Castor (Alpha Geminii), Murzim (Beta Canis Majoris) & Alphard (Alpha Hydrae) each with Magn.1.98.

Answer 8

They use a photometer. The brightest star-like object in the night sky is Venus. You'll probably notice it in the early evening sometime next year. Iridium flashes can be brighter than Venus. And the moon is the brightest of the nighttime objects (duh).



No, a streetlight is the brightest nighttime object. It clocks in at

-19.5 (totally guestimated figure from my bedroom window)

Answer 9

They don't exactly measure brightness. They try to calculate how big and how far away the star is. The bigger and closer the star, the brighter it is. The moon is the brightest object, generally.

Answer 10

They use "magnitudes". The brighter the star, the lower the magnitude.



There are two types of magnitude...

1. Apparent magnitude

which is the brightness seen from the earth

2. Aboslute magnitude

which is the brightness seen from 32.6 light years.



Well, if you see the moon, it is the brightest but if you don't..it might be Venus....

Answer 11

what kind of question is this?