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PHYS2220: Founda}Ons of Astronomy

PHYS2220: Foundaons of

• [email protected] • Course Text: Astronomy: A Physical Perspecve, Marc Kutner • 13 Lectures + 1 Class Exercise + 4 Tutorial Sheets + Final Exam – Final Exam = 50% – Tutorial Sheets = 10% each – Class Exercise = 10% each • New course component, exams will be similar to tutorial sheets. Lecture outline

1. Astronomical Coordinates: The Astronomical sky 2. Planetary Moon: The Solar System and Kepler’s Laws 3. Flux & Distance: magnitudes and

4. Stellar nucleosynthesis: Fusion, the CNO cycle 5. Properes of : The Hertzsprung-Russell diagram 6. Stellar lifecycle: Stellar rotaon and the Galacc Centre (SMBH)

7. The structure of our : Our Galaxy and its components

8. : morphological types, number of stars, space density, distribuon 9. Galaxy spectra: spectra, moon, inclinaon and peculiar velocies 10. Dark Maer: The virial theorem, galaxy rotaon curves, galaxy formaon

11. Black Holes and AGN: Black holes, Quasars, AGN Unificaon 12. Expansion: Olber’s Paradox, Hubble’s Law and the expanding Universe 13. Cosmology: The Cosmological Principle, the Hot Big Bang, Dark Energy The Astronomical Sky • On a human lifeme the Celesal Sphere is frozen • As the Earth rotates the celesal sphere appears to move – We can photograph trails (24hrs for one 360 degree rotaon)

• As the Earth orbits the – The stars visible in the night sky change over the course of the year (different constellaons become visible) Equatorial Coordinates

• At a fixed me in the Earth’s orbit we project our latude and longitude system onto the sky and call these and Declinaon – Longitude = Right Ascension: 0 – 24 hours [or 0 degrees to 360 degrees.] – Latude = Declinaon: -90 degrees to + 90 degrees

• This point in the Earth’s Orbit around the Sun is called “The first point of ” and is one of two *. – Vernal/Spring ~ 21st March – Autumnal Equinox ~ 21st September

• Because the Earth precesses (once every 26,000 years) the first point of Aries moves along the eclipc# (1 deg per 78 years). NB: nowdays Sun in during Vernal Equinox

• Astronomers work to either the 1950 or 2000 projecon denoted B1950.0 or J2000.0

*Equinox: When the Earth’s axis of inclinaon neither points towards or away from the sun. #Eclipc: The plane of the Earth’s rotaon around the Sun.

First Point of Aries

• At the moment of the Vernal Equinox the Sun will be directly overhead some point on the Earth (noon/midday).

• At this moment one can project a line of latude onto the sky and define this as 0 degrees right ascension. This projected line is known as the

• Right Ascension defined to increase East of this projecon.

• This grid is then frozen onto the Celesal Sphere and used to define posions of the fixed objects.

• E.g., M31: 00h44m37.99s, +41o16’9’’, J2000.0

E.g., What is overhead at 10pm tonight?

• On the Vernal Equinox: 0h RA is overhead at noon • Hence stars with coordinates ~12h will be overhead at midnight • The Earth completes one rotaon during the year • Whats overhead moves on by: – 1h per hr • Whats overhead at midnight moves on by: – 2h per

• 15th Sept is almost 6 aer the Vernal Equinox, hence tonight we will have stars with coordinates at ~0h overhead at midnight

• 10pm is two hours before midnight so we will have stars with coordinates 22h overhead at 10pm tonight.

Exercise 1 • Calculate what coordinates were overhead at the date/me of your birth?

• Path: – Calculate how many days your birthday is aer 21st March = N

– Add 12h+24hxN/365.25 = Q Because the sky completes one rotaon during the course of one year.

– Calculate how many hours before midnight you were born = M

– Subtract Q – M = P Because the sky rotates once per day

• This rule of thumb is good enough to plan what to observe on any night and any astronomer should be able to work out what is roughly overhead on any date/me as long as they can remember 12h overhead around 21st March. (Classic viva Q). Exercise 2

• What is the approximate locaon of the Sun in Right Ascension at 5pm on the 23rd of July. – Sun is at 0h RA on 21st March at noon – Sun is at 0h+24*(122/365.25) on 23rd July = 8.02h or 8h 00m 59.14s

• Note in this case we do not factor in any offset for the Earth’s rotaon about its axis. – The Sun’s posion in the Celesal Sphere changes because of the Earth’s orbit not its rotaon. – The chunk of sky overhead changes because of the Earth’s rotaon. Planning your observaons

• How long can I observe my target for?

– This depends on the target’s declinaon relave to your latude – Seeing and transparency degrade rapidly at sec Z > 2. – Wait ll star has risen to Z = 60 degrees, i.e. altude 30 degrees. – How much longer does it take to reach the meridian? – It will take the same me again from meridian crossing to seng below 30 degrees.

Z=Zenith angle, angle from zenith to towards horizon, 00=overhead The Meridian • A great circle divides the sky exactly into 2 hemispheres. • The meridian is the great circle that runs overhead through the zenith Z and the celesal pole P. – Angle PZ = 90 - latude φ

Z

P Right Ascension and Declinaon

• RA is the equivalent of longitude on the sky • Line of constant RA is a great circle running through star S and pole P. – Angle SP = 90 - declinaon, δ

Z

P

S Hour Angle • The meridian intersects with a line of constant RA through the star, at the pole. • The angle H = ZPS is the hour angle of the star. – Negave when star is east of the meridian – Posive when star is west of the meridian – Increases with me. Z

H P

S Altude, Zenith distance & Airmass

• Draw a third great circle through zenith Z and star S. • Angle ZS is the zenith distance z of the star. – Altude = 90 - z • Airmass – Airmass = sec z [where sec z = 1/cos z] Z 1.00 = overhead 1.41 = 45 degrees Zenith angle 2.00 = 60 degrees Zenith angle z H 3.00 = 70.5 degrees Zenith angle P

S Spherical triangles: the essenals

• All three sides are great circles. • Sine rule: sin a sin b sin c = = sin A sin B sin C

• Cosine rule: cosc = cos a cosb + sin a sin bcosC ! B

! c C

A When is star at a given zenith distance?

• We know the angles ZP, PS and ZS – need to know the hour angle H. cos z = cos(90 " #)cos(90 "$) + sin(90 " #)sin(90 "$)cos H % cos z = sin # sin$ + cos# cos$ cos H.

– So use cosine formula: Z ! z H P

S How many hours above 2 airmasses (altude of 30o)?

• Cosine formula gives H in degrees:

cosz !sin! sin" cosH = cos! cos"

– Divide H by 15 to convert to hours (i.e., 360 degrees = 24 hours) – Object rises above 2 airmasses when z=60 deg. – H gives me unl meridian crossing – Object is observable for twice this duraon Example

• The GAMA9hr field is at RA=9h and Dec=0o, esmate: – When would be the best date to observe this field? – If observed from the Anglo-Australian Telescope, how long would this object be at more than 30 degrees above the horizon per night? – If observed on the 1st Feb at what local me would this field rise above 30o and at what local me would it set below 30o?

• The AAT is at Siding Springs Observatory, NSW – Longitude =149.1o East of Meridian – Latude = 31.3o South of Equator – [Altude = 1150m above Sea Level] – not required for this queson Soluon to example • Knowns: – Z=60o – φ=-31.3o – δ=0.0o – RA=9h • Best me to observe field: – 12h overhead on 21st March (Vernal Equinox) – Sky moves on 2hrs per month – 9hrs overhead would be 1.5months earlier – Best me to observe would be approximately 7th Feb • Time above 30degrees: – Use formulae to find Hour angle (H): cosH=(cosZ-sinφsinδ)/cosφcosδ – cosH=(cos(60)-sin(-31.3)sin(0))/cos(-31.3)cos(0) – cosH=0.585 – H=54o – Earth rotates once every ~24hrs, i.e., 1hr=15 degrees – Therefore it will take 3.6hrs to rise from Z=60 to the meridian – And another 3.6hrs to fall from the meridian to Z=60 – Object will spend 7.2hrs above Z=30 • At what local me would the object rise and set on 1st Feb: – RA overhead on 7th Feb is ~9h (see answer to part1) – RA overhead on 1st Feb is ~8.5h (2hr per month so ~0.5hr per week) – Object therefore overhead on 1st Feb at half past midnight • Rises 3.6hrs earlier = 8.9pm or 8:54pm • Sets 3.6hrs later = 4.1am or 4:06am

Note: Precise answers will vary because: Equinox is not exactly midnight 21st March (varies each year), Earth takes slightly less than 24hrs to rotate, need to calculate offset due to Earth’s orbit precisely. However Astronomers should be able to make ballpark esmates like the above to plan their observaons and use online tools if they need greater precision. What me does a star rise/set?

• You’ve already worked out the hour angle H when the star rises above Z = 60 deg. – H is negave when star rises, posive when it sets.

• Rises/sets (>/< 30deg above horizon)

– Local standard me = RA - LSTmidnight+/-H60 • All values are approximaons but good enough to plan observaons

– In pracce many tools online to calculate them accurately for any object

– A good App for Macs is: iObserve Standard Times in Astronomy

Local Sidereal me (ST) = RA overhead now.

Most observatories have Sidereal clocks as well as local me clocks

Coordinated Universal me (UTC or UT1) = Time at Prime Meridian (GMT)

Standard Time = wristwatch me! (GMT+7 in Perth)

What’s the me is suddenly not such an easy queson!

Sidereal Time

Sidereal day = 23hrs 56m 4s

Solar day = 24hrs

Siding Springs Observatory

Anglo-Australian Telescope 3.9m

Built 1963

Responsible for 40% of all known redshis

Australia’s main onshore opcal observatory Warrumbungle Naonal Park Near Coonabarabran Anglo-Australian Telescope (AAT)

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