stars in the night sky - university of virginia · stars in the night sky stars are “suns” but...
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Stars in the Night Sky
Stars are “suns” but typically millions of times further away than our own Sun.
A few thousand stars are visible to the unaided eye But only if you are observing from a “dark” location
What you can actually see depends on the brightness of the sky and the quality of your vision.
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Stars in the Night Sky
Most people live in heavily light-polluted locations
http://www.inhabitat.com/2006/08/19/light-pollution-the-continuing-spread/
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Stars in the Night Sky
Most people live in heavily light-polluted locations
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Stars in the Night Sky
Even far from city lights our insensitive eyes see only a fraction of the 100 billion stars in our galaxy.
Although if you count the Milky Way itself, we see quite a few...
The individual stars that we see are all part of the Milky Way galaxy and tend to be the nearest stars to us.
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Stars in the Night Sky
The stars we see with our eyes maybe be intrinsically bright and far away or faint and nearby.
Said another way, two stars of equal apparent brightness may be at significantly different distance from us.
Apparent groupings of stars are typically physically unassociated.
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The Magnitude System
The stars that we can see span a range of apparent brightness. The ancient Greeks identified the brightest stars as the “first”
magnitude and ranked successively fainter stars down to the “sixth” magnitude – the faintest visible to the eye.
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The Magnitude System
Modern Astronomers have created a quantitative magnitude system approximating the original Greek ranking system
Stars are ranked from brightest to dimmest by magnitude That is, stars with numerically larger magnitudes are fainter
− A 5th magnitude star is fainter than a 1st magnitude star Five magnitudes difference is a factor of 100 difference in apparent
brightness− A 12th magnitude star appears 100 times fainter than a 7th magnitude star− A 5th magnitude star is 1,000,000 times brighter than a 20th magnitude star− 20-5 = 15 = 5 + 5 + 5 => 100 x 100 x 100 = 1,000,000
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The Magnitude System
Following the trend of brighter objects having numerically smaller magnitudes, extremely bright objects (for example Venus) can have negative magnitudes.
– The brightest star in the sky, Sirius, is a -1.5 magnitude star.
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The Magnitude System
Modern Astronomers have created a quantitative magnitude system approximating the original Greek ranking system
Stars are ranked from brightest to dimmest by magnitude That is, stars with numerically larger magnitudes are fainter
− A 5th magnitude star is fainter than a 1st magnitude star Five magnitudes difference is a factor of 100 difference in apparent
brightness− A 12th magnitude star appears 100 times fainter than a 7th magnitude star− A 5th magnitude star is 1,000,000 times brighter than a 20th magnitude star− 20 - 5 = 15 = 5 + 5 + 5 => 100 x 100 x 100 = 1,000,000
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Constellations
Classically the constellations are patterns of stars associated with characters of myth and legend.
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Constellations
Classically the constellations are patterns of stars associated with characters of myth and legend
Constellation Mythology
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Constellations
Formally/scientifically the constellations define 88 regions of the sky the same way the 48 states divide the continental US.
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Asterisms are not Constellations
Some commonly recognized patterns of stars have names but are not constellations
The Big Dipper is a prime example, being just the rear end and tail of Ursa Major (the Great Bear)
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Star Names
Every star you eye can see has dozens of names The bright star Vega, overhead at sunset in September is also
α Lyrae, HR 7001, 3 Lyrae, 2MASS J18365633+3847012, …
The brightest stars have classical names, originating largely from the Arabic language.
Rigel (β Ori) – The left leg of the giant
Sheliak (β Lyr) – The tortise
Aldebaran (α Tau) – The Follower (of the Pleiades)
Antares (α Sco) – The rival of Mars
Stars are named, within a constellation by Greek letter in order of decreasing brightness (α, β, γ, δ....
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Star Names
Every star you eye can see has dozens of names The bright star Vega, overhead at sunset in September is also
α Lyrae, HR 7001, 3 Lyrae, 2MASS J18365633+3847012, …
The brightest stars have classical names, originating largely from the Arabic language.
Rigel (β Ori) – The left leg of the giant
Sheliak (β Lyr) – The tortise
Aldebaran (α Tau) – The Follower (of the Pleiades)
Antares (α Sco) – The rival of Mars
Stars are named, within a constellation by Greek letter in order of decreasing brightness (α, β, γ, δ....
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Star Names
Every star you eye can see has dozens of names The bright star Vega, overhead at sunset in September is also
α Lyrae, HR 7001, 3 Lyrae, 2MASS J18365633+3847012, …
The brightest stars have classical names, originating largely from the Arabic language.
Rigel (β Ori) – The left leg of the giant
Sheliak (β Lyr) – The tortise
Aldebaran (α Tau) – The Follower (of the Pleiades)
Antares (α Sco) – The rival of Mars
Stars are named, within a constellation by Greek letter in order of decreasing brightness (α, β, γ, δ....
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Star Names
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Star Names
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The Visible Constellations Change Throughout the Year
What we see of the Universe at night depends on where the Earth is relative to the Sun.
Our midnight sky is the direction in space opposite the direction to the Sun
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The Visible Constellations Change Throughout the Year
What we see of the Universe at night depends on where the Earth is relative to the Sun.
As the Earth orbits the Sun our perspective on the night sky changes throughout the year.
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The Visible Constellations Change Throughout the Year
What we see of the Universe at night depends on where the Earth is relative to the Sun.
The Earth moves enough in one day to advance the setting time of a particular star by about 4 minutes - 2 hours a month.
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The Visible Constellations Change Throughout the Year
What we see of the Universe at night depends on where the Earth is relative to the Sun.
At 8pm one month from now the sky will look as it does at 10 pm today.
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Constellations Change with Time
The stars are all moving relative to one another These motions are perceptible to the eye only over thousands of years.
The constellations/asterisms known to us will be obsolete 100,000 years from now.
Movie of the Changing Big Dipper