Topic 3 Flashcards
What do stars look like?
Stars look like points of light in the sky
They twinkle towards the horizon
They are very far away
What do true binaries look like?
True binaries are double stars that orbit each other
What do optic doubles look like?
Optic doubles are double stars that look like two very close stars, but are in fact far from each other
What do open clusters look like?
Open clusters are groups of stars that are close to each other in space.
They are usually very bright
Young stars are often found in open clusters
What do globular clusters look like?
Globular clusters are more packed than open clusters
Often spherical
Have a large concentration of stars towards the cluster’s nucleus
Old stars are usually found in globular clusters
What do nebulae look like?
Nebulae are faint, blurry, fuzzy patches of light in the sky
What do asterisms look like?
Asterisms are patterns of stars in the sky (eg the Plough)
What do constellations look like?
Constellations are a recognisable pattern in the sky.
There are 88 official constellations
How are stars labelled according to their brightness?
They are named using a Bayer Designation
The brightness is indicated by a greek letter, with the brightest star being “alpha”, followed by the constellation the star is in.
e.g. Alpha Centauri
How was the official list of constellations established? Is this list the same for all cultures?
The original list was written by Ptolemy, who based the constellations around Greek mythology.
The list was developed until eventually, the IAU designated 88 official different constellations.
Asian and Latin American cultures have different constellations
(e.g. Capricornus is a sea goat in the West, but an antelope for the Indians)
Can you draw the Plough, Orion, Cygnus and Cassiopeia?
refer to http://tinyurl.com/constellationdrawings
What are the pointer stars in Ursa Major, and where do they point to?
The two stars on the end of the “pan” shape of the plough in Ursa Major point to Polaris
The “handle” of this “pan” can be used to find Arcturus
What are the pointer stars in Orion, and where do they point to?
Orion’s belt points westwards and down to Sirius, and eastwards to Aledebaran and the Pleiades Cluster
What are the pointer stars in Pegasus, and where do they point to?
The top left star in the Pegasus square points to the Andromeda Galaxy
The bottom right star of the Pegasus square points to Formalhaut
Can all constellations be seen all year round?
No, only circumpolar constellations can. Some constellations are seasonal, and so are blocked by sunlight when they are out, and some are blocked by the sun as they are too close to the ecliptic
What is “right” ascension and “declination”?
Right ascension is the distance eastwards from the celestial prime meridian - measured in hours & minutes
Declination is the degrees north or south from the celestial equator
Why does Polaris appear fixed in the night sky?
Polaris appears fixed in the night sky as it is a pole star, with a latitude of 90 degrees, meaning it lies very close to the axis of the Earth’s rotation
What is the Altitude of Polaris equal to?
The Latitude of the Observer
What is a circumpolar star, and how do you work out if a star is circumpolar?
A circumpolar star is one that is always above the horizon and never sets below your horizon.
A star is circumpolar if its declination > 90 - latitude
Deneb has a declination of 45 degrees. Will it be circumpolar from London?
Deneb’s declination = 45 degrees
Is it circumpolar from London?
45 > 90 - 51
Deneb is not circumpolar from London as 39<45
Alpha Centauri has a declination of -61 degrees. How far south would you have to be to see it as a circumpolar star?
Alpha Centuri’s declination = -61 degrees (61 degrees South)
61 > 90 - 29
Therefore you would need to be at a latitude of 29 degrees South
What is a long exposure photograph, and how can one use it to find the length of a sidereal day?
A long expose photograph will produce a picture showing star trails.
By measuring the time taken for a full circle in a circumpolar star in a star trail, you can find the length of a sidereal day by doing:
Rotation Period of Earth / Exposure Time =
360 / Mean Arc Angle
What is a planisphere and how do you use it?
A Planisphere consists of two discs fastened to each other
Around the edge are the hours of the day
On the bottom disc is a star map showing the constellations
The edge of the round area is the horizon
e.g. to find the stars available on the 4th of September at 10pm, you would move the hour dial on the overlay next to the date on the underlay and point the planisphere north to see what stars would be in the sky
What is the ecliptic and the zodiacal band?
Ecliptic: the path of the apparent movement of the sun
Zodiacal band: a region of the sky +/- 8 degrees of the ecliptic. It is where we find the path of all the planets
What equipment would you need for a naked-eye observation session and why?
Red torch - provides lighting without deacclimatizing the eyes to darkness
Clipboard: to sketch constellations whilst standing
Pencil/rubber: to record observations
Warm clothes: nightly observations are often cold, so warm clothes are needed to insulate you
What are the three main naked-eye observation techniques?
Dark adapted eyes: when eyes become adapted to darkness after not looking at light for approx. 30mins
Averted vision: looking slightly to the side of a faint object to see it better
Relaxed eyes: easier to observe when the eyes are not strained
What is the Messier Catalogue and how can we identify Messier Objects?
The Messier Catalogue was a catalogue of 110 celestial objects, previously thought to have been comets.
They can be identified due to them being prefixed with ‘M’ (e.g. Crab Nebula is M1)
Why do stars seem to move from east to west in the night sky?
Stars appear to move west to east because of the anticlockwise rotation of the earth.
Because the earth rotates eastwards the sun and stars rise and move from the east to the west.
When do stars cross the observer’s meridian and culminate?
A star culminates and so crosses the observer’s meridian when it is due south (fancy way for saying it culminates), and it is at its highest altitude.
Are stars in constellations and clusters attracted to each other?
Stars in a constellation have no relation to each other
Stars in a cluster are gravitationally linked
What is the difference between optical doubles and binary stars?
Optical doubles - appear to be close to each other but have no relations whatsoever
Binary stars may be 2+ stars orbiting each other
What is the scale of apparent magnitude?
Apparent magnitude is how bright a star appears to be
The apparent magnitude scale is logarithmic, whereby 6 is x100^(1/5) brighter than seven
It is represented by a lowercase ‘m’
What is the difference in brightness between stars where m=1 and m=6?
the difference is (100^(1/5))^5 = 100
How is heliocentric parallax used to determine the distances to nearby stars?
We can measure the distance to nearby stars by measuring how much they appear to move against distant stars. We measure it in one month and then again six months later when the Earth is on the opposite side of the Sun.
The greater the parallax angle, the closer the star.
This can only be used on stars closer than 30 parsecs away.
How big is one parsec (pc)?
1 parsec = 3.26 light years
What is absolute magnitude?
How bright a star would appear if it was 10pc away
What is the law of light intensity?
The brightness of a star has an inverse square relationship with its distance (i= 1/d^2)
What is the distance modulus formula?
M = m+5 -5 log d
