Download Introduction to Astronomy - Northumberland Astronomical Society

Introduction to Astronomy
Part 3 — Window on the Universe
Dr Adrian Jannetta FRAS
Northumberland Astronomical Society
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Summary
This presentation deals with the systems astronomers use to
measure position in the sky, apparent size, distance and
brightness of objects.
Size Angular diameter (and object comparisons).
Position Horizon and equatorial coordinate systems.
Distance Astronomical Unit (AU), light-years and parsecs.
Brightness The magnitude scale.
We’ll also examine how our location on the Earth affects the
stars and constellations we can see and how changes in the tilt
of the Earth’s axis change the view over long time periods.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Angles in astronomy
An angle is a measure of rotation.
Astronomers often need to give the positions of objects in the sky or
‘distances’ from one part of the sky to another.
Angles are an appropriate way to do this! For example:
The Sun travels an angular distance of 360◦ per year around the sky.
The angular distance between the horizon and overhead is 90◦ .
The apparent sizes of celestial objects can also expressed using an angle.
It’s useful for amateur astronomers to be familar with angles.
Estimating angular sizes
With your hand at arms length, this shape makes an angle of 15◦ ,
which encompasses the brightest stars of Cassiopeia.
Estimating angular sizes
With your hand at arms length, this shape makes an angle of 10◦ .
This is about the distance between Betelgeuse and the three Belt
Stars in Orion.
Estimating angular sizes
With your hand at arms length, this shape makes an angle of 5◦ . This
is about the distance between the pointer stars (Dubhe and Merak) in
The Plough.
Estimating angular sizes
◦
With your hand at arms length, this shape makes an angle of 12 .
This is diameter of the lunar disk. Some other objects of comparable
size are also shown.
Small angles
1◦ = 600 (1 degree equals 60 arcminutes).
10 = 6000 (1 arcminute equals 60 arcseconds).
Crescent moon and Venus on May 21st 2004. Venus had an angular diameter of 50
arcseconds (just under 1 arcminute). The lunar diameter was about 30 arcminutes.
Very small angles
Extremely small angles
Pluto has an angular diameter of about 0.100 but astronomers are pushing
the boundaries to smaller angular diameters!
The Hubble Space Telescope can resolve detail to less than 0.1 arcseconds
(100 milliarcseconds!), whilst an Earth-based 10 metre class telescope can
attain a resolution of about 20-50 milliarcseconds).
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Units of distance
Miles and kilometres are only useful on planet-sized scales.
Astronomical Unit (AU) Unit of distance based on the mean
Earth-Sun distance.
1 AU = 149, 597, 871 kilometres = 92, 955, 807 miles
A convenient unit for distances on the scale of the solar
system.
Light-year (Ly) Based on the distance that light (in a vacuum) can
travel during one (julian) year.
1 Ly ≈ 10 trillion kilometres ≈ 63, 241 AU
Convenient for distances within and beyond the galaxy.
Parsec (pc) The professional astronomer’s unit of distance.
1 pc ≈ 3.26 Ly
The parsec
The parallax angle p is tiny!It is
defined by
p
=
=
radius of Earth orbit
Star distance
1
d
Rearranging this gives a way of
determing the distance if we can
measure the angle p.
d=
The pasec is defined using the size
of the astronomical unit (in picture).
A parsec is the distance from the
Sun to an object which has a
parallax shift of 1 arcsecond.
1
p
For example, for the star Procyon
the shift is p = 0.28500 ,which gives
d=
1
= 3.51 pc
0.285
This distance is about 11 light-years.
Horizon coordinates
Every point on the sky can be specified by two numbers:
Altitude an angle between 0◦ (on the horizon) and 90◦
(overhead).
Azimuth an angle measured from North (0◦ ), through East
(90◦ ), South (180◦ ), West (270◦ ) and back around
to North (360◦ /0◦ )
Celestial objects rise and set; their horizon coordinates are
always changing. Horizon coordinates will vary with location on
the Earth (e.g. the object will appear higher or lower in the
sky).
Horizon coordinates simulator
Click to start
Courtesy of UNL Astronomy Education
Equatorial coordinates
Every point on the sky can be specified by two numbers:
Declination an angle measured north or south of the celestial
equator. The North Celestial Pole is at +90◦ and
the South Celestial Pole at −90◦ .
Right Ascension an angle measured from a zero line (the First
Point of Aries) to the object line. The RA of an
object is usually expressed as the time period
between the zero line being on the local meridian
to the object being on the meridian.
The RA and Dec of stars doesn’t change much over human
lifetimes - almost constant.
Equatorial mounts of telescopes are designed to follow this
system.
Right Ascension and Declination simulation
Click to start
Courtesy of UNL Astronomy Education
Equatorial and Horizon coordinates
Our view of the celestial sphere is dependent on our position on the Earth.
Click to start
Courtesy of UNL Astronomy Education
The effect of changing latitude
(a) 55◦ N
(b) 45◦ N
(c) 35◦ N
Travelling south causes constellations in the north to sink towards the
horizon.
The Ecliptic
The Sun traces out a circular path in the sky called the ecliptic as the
Earth orbits around it.
Stars to the east are visible in evening sky.
Stars to the west are visible in morning sky.
Stars opposite the Sun are visible all night.
Seasonal changes
Our orbit around the Sun causes a slow drift in the visibility of the
stars and constellations. The cycle repeats annually.
(d) April 1st 2330UT
(e) August 1st 2330UT
Axial precession
Hipparchus (190BC –
120BC) noticed a small
difference in the measured
positions of stars made by
earlier observers.
The direction of the Earth’s
rotation axis is slowly
changing with time.
The axis traces out two
cones over a period of 26,000
years.
Like a spinning top!
Caused by the gravitational
pull of the Sun and Moon.
Changing pole stars
Axial precession changes the direction of the rotation axis and changes the Pole
Star over long enough periods. Many equatorial mounts will eventually need new
polar scopes!
Changing constellations
(a) April 2012
(b) April 6500 BC
Precession brings new constellations into our sky if we wait long enough. The
Southern Cross (Crux) used to be visible from the UK. It will be again in the
future.
Magnitude scale for brightness
The magnitude scale of brightness is based on a simple six point
scale devised by Hipparchus.
Brightest stars — magnitude 1
Faintest stars — magnitude 6
The definition was put on a more precise footing in the 19th
century (by N R Pogson).
Difference of 5 magnitudes corresponds with exactly 100
times difference in the amount of light.
Logarithmic
scale; One magnitude difference is
√
5
100 ≈ 2.51 times brightness difference.
The star Vega was chosen to be magnitude 0.
Scale extended to include brighter and fainter objects than
the scale was designed to include.
Magnitudes of some common objects
The following scale shows how the magnitude scale quantifies the brightness
of some objects in the night sky visible without optical aid.
Sirius
-2
-1
Vega
Spica
Polaris
0
1
2
Andromeda
Galaxy
(M31)
3
Naked
eye
limit
Uranus
4
5
6
7
The magnitude scale must be extended to include brighter objects than the
stars and fainter objects than those visible with the naked eye.
Bright
Iridium
flare
Sun
-30
-25
Full moon
-20
-15
-10
Sirius
Venus
-5
0
Naked
M31 eye
limit
5
Pluto’s
smaller
moons
Pluto
10
15
20
25
HST
visible
light
limit
30
How deep can you see?
The magnitude limits for some binoculars and telescopes are given
below.
The diameter of the objective lens (or mirror) determines how much
light is collected.
Instrument
Naked eye
Binoculars (8 × 30)
Binoculars (10 × 50)
Telescope (2 inch / 50mm)
Telescope (4 inch / 100mm)
Telescope (6 inch / 150mm)
Telescope (10 inch / 250mm)
Telescope (16 inch / 406mm)
Magnitude limit
+6.5
+9.5
+10.5
+11.7
+12.7
+13.4
+14.1
+14.4
These values are approximate; local observing conditions, health, age
and experience will influence them to some extent.
These values don’t apply to imaging - only visual observations.
Test yourself!
Decide if the following objects are visible with the equipment
you have.
Name
Type
R.A.
Dec.
Mag.
Size.
Const.
Omega Centauri
Globular cluster
13h 27m
−47◦ 290
+3.7
360
Centaurus
A bright star cluster - easily visible to the unaided eye. Bigger in size
than the full moon! The declination value indicates that it’s only
visible south of about 42◦ N. Too far south for the UK.
Test yourself!
Decide if the following objects are visible with the equipment
you have.
Name
Type
R.A.
Dec.
Mag.
Size.
Const.
3C 273
Quasar
12h 29m
+02◦ 030
+12.8
Stellar
Virgo
This is the brightest quasar in the sky and it’s visible from the UK. It
looks like a star and it’s faint magnitude means a telescope is needed
to see it.
Test yourself!
Decide if the following objects are visible with the equipment
you have.
Name
Type
R.A.
Dec.
Mag.
Size.
Const.
NGC 6543
Planetary Nebula
17h 59m
+66◦ 380
+8.3
2200
Draco
This nebula is known informally as the Cat’s Eye. It’s bright enough
to be visible with binoculars but it’s tiny size (about the size of
Saturn) will require high magnification from a telescope! An easy
target from the UK as it is so far north of the celestial equator that it
never sets.