Unit 10: Invisible Astronomy

What does the electromagnetic spectrum look like?

What are some radio wave advantages?

• detected all day

• detected on ground

• reflected off metal

• not absorbed by dust

• emitted by distant young galaxies

What explains the fact that radio waves can be detected all day?

• long wavelength

  • impervious to atmosphere & cloud molecules

• Sun produces low radio emission

What explains the fact that the atmospheric opacity of gamma ray, X-ray, and UV waves is at 100%?

• smaller particles than atmosphere

  • bounce away

What number is the approximate atmospheric opacity of UV waves becoming visible?

80%

What number is the approximate atmospheric opacity of visible light?

10%

What explains the fact that big telescopes are on mountains?

• more thin atmosphere = more light

What explains the fact that infrared waves can touch the ground?

• atmosphere contains certain molecules that efficiently block in specific spots

What number is the approximate atmospheric opacity of microwave & radio waves?

0%

What number is the limit to radio wavelengths that touch the ground?

50m

What explains the fact that radio waves beyond 50m don’t touch the ground?

reflect off upper atmosphere

What are commonly found in distant galaxies?

radio jet

What did military start to use leading to WWII?

radar

What was accidentally detected in the early 1930s by Karl Jansky?

1st radio light

What explains the method of discovering radio light?

• Jansky worked at Bell Labs

• investigated transatlantic telephone by radio waves

• identified radio sources interfering with voice

What radio noise did Jansky find?

• thunderstorm

• sky hiss

  • rising & falling in strength every 24 hours

What did Jansky initially believe about the radio hiss?

come from Sun

What did Jansky finally believe about the radio hiss?

come from Sagittarius constellation / Milky Way disk

• because Milky Way centre rises & sets

What was announced on May 5, 1933 in the New York Times?

Jansky’s discovery

• “New Radio Waves Traced to the Centre of the Milky Way”

What did Grote Reber do in 1937?

built 10m dish in backyard by iron sheet

What did Grote Reber identify in 1937?

discrete radio emission

What was a diffuse radio source in Reber’s map?

Milky Way centre

What was a discrete radio source in Reber’s map?

2 unknown

What did Dutch astronomer Hendrik van de Hulst predict in 1945?

• H naturally emits 21 centimetre radio wave when electron flips

What are “aligned poles?”

• high E state

  • unnatural repulsion force

• stable for 10 000 000 years

What are “opposite poles?”

• low E state

  • natural attraction force

What process causes opposite poles?

1. electron flips

2. emits 21 centimetre wavelength energy

What was astronomers’ response to Hendrik van de Hulst’s prediction in the 1950s?

• 21 centimetre emission

  • strong along Milky Way disc

What did astronomers find about hydrogen in our Galaxy?

• 21 centimetre radio emission map = Galaxy hydrogen map

What was the key result of the Dutch astronomers’ 21 centimetre map in 1958?

• H is arranged in discrete locations

→ spiral arms

What is Earth’s Universe address?

• Earth

• Solar System

• Orion Spur Arm

• Milky Way

• Local Group

• Local / Virgo Supercluster

What is the nearest discrete radio emission source?

Sun

What did English radar engineer Stanley Hey detect in 1942?

• radio from solar flare

What is a “solar flare?”

• radiation eruption tracing magnetic field line from sunspot

What caused Stanley Hey to detect radio from solar flare?

• English radar sites tracking enemy planes were interfered by radio emission

What did Stanley Hey initially conclude about the interfering radio emission?

enemy defence

What did Stanley Hey finally conclude about the interfering radio emission?

radio surge matched solar flare

→ related to Sun magnetic field

What happens what atoms fly through magnetic fields?

radiate & trace magnetic field line

What explains the Sun’s differential rotation?

• fluid body

What is “differential rotation?”

• rotation speed decreases with latitude

  • closer to equator = quicker spin

What explains Earth’s magnetic field being a big bar magnet?

• core = spinning iron sphere

What is Sun’s magnetic field?

un/twisting field every 11 years

What did German astronomer Samuel Schwabe track over 2 decades?

• visible sunspot number reaches hundreds from handfuls every 11 years

What is a “solar minimum?”

• few sunspots

What is a “solar maximum?”

• many sunspots

What process causes sunspots to appear in pairs?

1. magnetic field comes out surface

2. loses heat

3. magnetic field goes in surface

What was discovered about Sun radio emission in the 1850s?

• magnetic storm & aurora followed Sun cycle

What causes magnetic storms & auroras?

• charged particles by sunspots

What are the name origins of “aurora?”

• Roman dawn god

What are the word origins of “borealis?”

• the south

  • Latin

What is a “magnetic storm?”

• sudden & temporary magnetic field scramble

What causes magnetic storms?

• solar wind of charged particle distorts Earth magnetic field

What is an “aurora?”

• charged solar wind particle glow

What causes auroras?

• solar charged particle wind follows Earth magnetic field to pole & smash Earth atmosphere

What is a “radio array?”

• multiple radio dishes

What detected the most distant observed object?

radio array

What are radio arrays’ advantages?

• cheap

• practical

What is the “SKA?”

• Square Kilometre Array

  • radio array with most dishes

  • see 50x further

What did English radio astronomers Jocelyn Bell Burnell & Antony Hewish detect from supernova remnant cores in 1967?

• pulsar

What did Jocelyn Bell Burnell & Antony Hewish initially conclude about the radio source that pulsed radio emission?

• intelligent life

What did Jocelyn Bell Burnell & Antony Hewish finally conclude about the radio source that pulsed radio emission?

• radio light pulse

What is a “pulsar?”

• neutron stars pulsing radio light

What produces magnetic fields?

• spinning charged particles

What causes pulsars?

1. neutron star sucks charged particles from gas cloud

2. … accelerates

3. … shoots out radio beams

4. radio beams sweep circle

   • magnetic axis is not always aligned with spin axis

5. … aim at Earth

What was found about binary pulsars in the 1980s?

tick slower when close to centre of mass

What is “time dilation?”

• time slows down in strong gravity field

What explains time dilation?

more gravity = slower time

What explains why one would see the opposite of GR’s prediction if pulsars spin slower due to star gravity?

• gravity causes pulsar to speed up

What did Stanley Hey identify about radio galaxies & quasars in 1946?

• 1st radio source from galaxy

What was the 1st radio source from galaxy?

Cygnus A

What is a “radio galaxy?”

• (typically) giant elliptical with radio jet

What causes radio jets?

• galaxy nucleus

What is a “Seyfert galaxy?”

• radio- loud / bright spiral

What did American astronomer Martin Schmidt investigate in the 1960s?

• star-like spectra at same coordinates as strong radio sources

What are “quasars?”

• Quasi-Stellar Radio Source

  • huge redshifted galaxy

What is the “Doppler Effect?”

• larger shift = faster object

What is “Hubble’s Law?”

• higher recessional speed = more distant object

What causes quasars?

• bright star-like appearance = huge swirling matter

What was proposed about binary star system black holes?

• black hole can accrete companion star matter & form swirling disc around black hole

• black hole’s spin produces magnetic field

  → radio lobe

What is an “accretion disc?”

• disc around supermassive black hole at galaxy centre

What would the space immediately surrounding a black hole have?

near infinite gravity

What do radio galaxy images reveal?

• huge hot gas & dust disc around dark object

What explains why only distant quasars are seen?

• young phase

  • denser galaxy core

    → more black hole fuel

  • accretion disc disappears

    → stable galaxy

What do nearby galaxies emitting radio jets suggest about black holes?

• temporarily reactivated

What causes temporarily reactivated black holes?

1. galaxies interact

2. gravitation pull pushes stuff towards hibernating black hole

What explains why most galaxies have a quiet central black hole?

• consumed all nearby matter

What is “Sagittarius A?”

• brightest Galaxy radio source

What is the Milky Way supermassive black hole size?

6 light hours

• 1 000 000 000 kilometres

What explains why x-ray has to be observed from space?

• can’t penetrate Earth atmosphere

What did NASA launch in the late 1970s?

• Einstein Observatory

  • 1st x-ray space observatory

What is the strongest galactic x-ray source, discovered in the 1960s?

Cygnus X-1

What causes accretion discs to glow?

• high speed

What is a “stellar mass black hole?”

• black hole from single dead star

What is in the most steeply curved space around the stellar mass black hole?

x-ray

• high energy radiation

What is the process of detecting black holes?

1. search for companion star

   • bright x-ray source

     • accretion disc

What did Canadian astronomer Tom Bolton find about Cygnus X-1 in 1972?

• binary system