LinR: Stars: Distances, Magnitudes and Types

Credit: ESO, Stefan Gillessen (MPE), F. Eisenhauer, S. Trippe, T. Alexander, R. Genzel, F. Martins, T. Ott

If the stars should appear but one night every thousand years how man would marvel and stare. Ralp Waldo Emerson

Preamble: on the naming of things!

The brightest stars have names that derive from (usually) Arabic: e.g. Ursa Major

• Subsequently stars named with Greek letters, in order of brightness:e.g. Orion α Orionis = Betelgeuse β Orionis = Rigel (Unfortunately, Rigel is brighter than Betelgeuse, since it is much hotter & radiates mainly in UV! Therefore, this system refers to visual brightness only.)

  Credit & Copyright: Matthew Spinelli

Messier

The most useful catalogs are ones of specific objects: e.g. Messier (pr. Messié!) the most famous catalog consists of things that aren't comets(!): M1 = Crab nebula M3 = Globular cluster M31 = Andromeda galaxy M45 = Pleiades cluster M51 = Spiral galaxy M57 = Ring nebula

Credit & Copyright: P. Gitto

Brightness/Magnitude

Easiest observation about stars is that some are brighter than others.
Hipparchus defined brightest to be of first magnitude, down to the dimmest of sixth magnitude. A first mag. star turns out to be 100xbrighter than a 5th mag.

• This seems backwards (it is!): a twentieth magnitude star is .00000000001 times as bright as a first magnitude star (sigh!).
• Furthermore, we need to have negative magnitude stars which are brighter than positive magnitude stars (double sigh!)
• Finally we have close dim stars (like Sirius) which are much brighter than distant bright stars (like Rigel) (triple sigh!)

before we get started,

Some Numbers:

• Distance:

  Popular measure is the light year: distance traveled by light in 1 yr ~ 10¹⁶m.
  Astronomers usually use the "parsec": 1pc = 3x10¹⁶ m (thirty trillion km). Closest star (α Centauri) is at a distance of ~1.3 pc. Sirius is at about 5 pc.

• Time:

  One million years (1 Myr) is fairly short

• Speed:

  Speed of light is the standard (300000 km/s). Low speeds are 100 km/s.

• Mass:

  Sun has mass of 2*10³⁰ kg: will refer to this as M_o (so Earth has a mass of ∼ one millionth M_o)

• Brightness/Luminosity:

  Sun has (absolute) magnitude 5
  Brightest normal stars are about -8, dimmest things we can see are about 26

Stellar evolution once over lightly:

(note in passing: we talk about stellar evolution, which is stupid, since we don't talk about the evolution of a baby into an adult.)

Also note: ALL stars go through ALL the stages: the reason why we only see (e.g.) 100 or so planetary nebulae vs 10⁸ normal stars is that the lifetime of a planetary nebula is only ≈ 50000 yrs, vs 10¹⁰ yrs for a main sequence star

• Solar mass stars: Times are approximate in years.

• Large Mass stars Endpoint depends on mass of star: 5M₀ ⇒ neutron star 20 M₀ ⇒ black hole Note lifetimes are much shorter.

• Note that big stars live fast, die young

  My candle burns at both ends
     It will not last the night,
  But ah my foes and oh my friends,
     It gives a lovely light.
  

  Edna St Vincent Millay

1. young stars:
2. Star Death
3. Star Remnants

Star Birth

Note Left centre: group of young stars Left lower:: star still blowing away cocoon of gas Right: glowing "lanes" of gas heated by large stars Centre left: dark lanes where stars are about to form

Credit & Copyright: T. Rector (U. Alaska Anchorage), Gemini Obs., AURA, NSF

Eagle Nebula: Cluster of stars just formed in centre of dark shell of dust and gas, taken with the 0.9-meter telescope on Kitt Peak, Arizona, USA. Part of M16

Credit & Copyright: T. A. Rector & B. A. Wolpa, NOAO, AURA,

Eagle close up: pink light is sulphur. Young stars excite the gas so it glows around the "birth pillars". Large stars will go supernova in about 5 million years

Credit: P. Challis (CfA), Whipple Obs., 1.2 m Telescope

The Eagle's EGGs: evaporating gaseous globules (EGGs). Very dense parts of the Eagle contract to form new stars which promptly blow away the surrounding dust and illuminate the columns

Credit: J. Hester, P. Scowen (ASU), HST, NASA

Henize 206: Another star forming region in the Large Magellanic Cloud. Can see left over remnants of old supernova at the top, which compressed the gas and triggered the star formation

Credit: V. Gorjian(JPL) et al., JPL, Caltech, NASA

N81: a group of very young hot stars in the Small Magellanic Cloud heating up the nebula round them

Credit: M. Heydari-Malayeri (Paris Obs.) et al., Hubble Heritage Team, NASA

XZ Tauri consists of 2 very young unstable stars, separated by about Sun-Pluto distance, emitting vast cloud of gas (pictures taken over 5 years)

Credit: John Krist (STScI) et al., WFPC2, HST, NASA

The best known cluster is the Pleiades: (Seven Sisters except we can only see 6 now)

A closer look: the Pleiades are a very young group (∼ 50) of stars, about 107 years old, and very close: about 40 light-years, so light takes 40 years to travel from them. Note the "star-stuff" still blowing away.

Spectral Types, Sequence

Cannot measure energy output of most stars directly (too little!) hence can't measure temp via BB curve as we did for the sun. However spectra vary from one star to next.

Vega

Aldebaran

Note hotter (bluer) stars show more H, less complicated spectrum

This provides a direct measure of the surface temp of the star. This allows us to classify stars according to their spectra

Stellar Spectral Types: OBAFGKM Credit & Copyright: KPNO 0.9-m Telescope, AURA, NOAO, NSF

These are subdivided into 10 smaller classes running from 1 to 10: e.g. the sun is a G2 star.

• A mnemonic

Star Death

If stars are small ...

The classic example is M57: The Ring Nebula Central star is a white dwarf (50000°) Hot blue gas at centre coolest red gas along the outer boundary.

Credit: H. Bond et al., Hubble Heritage Team (STScI /AURA), NASA

The star blows away its outer layers, so almost all the older ones we knew look like this.

• or like this

• This is the Cats-eye nebula: looks like successive explosions

Mz3: The Ant Nebula. Probably magnetic field is creating a "focussed" planetary nebula

Credit: R. Sahai (JPL) et al., Hubble Heritage Team, ESA, NASA

Planetary Nebula CRL 618: this was a red giant a few hundred years ago, but it is now expelling jets of gas

Credit: Susan R. Trammell (UNC Charlotte) et al., ESAIC, HST, ESA, NASA

NGC 2440: a very hot white dwarf which is blowing off its outer layers much faster

Credit: H. Bond (STScI), R. Ciardullo (PSU), WFPC2, HST, NASA

IC 4406: a really weird planetary nebula: probably a cylinder that we see side on. How can a round star make a square nebula? IC 4406 is most probably cylindrical, with its square appearance the result of our vantage point.

Credit: H. Bond (STScI), R. Ciardullo (PSU), WFPC2, HST, NASA

Supernova

If Stars are large....

Approx 1/30 yr known in Milky Way
6 visible in recorded history

1006 Type I SN 1006: History's Brightest Supernova. THis shows remnants of the expanding shockwave

Credit: Frank Winkler (Middlebury College) et al., AURA, NOAO, NSF

1054 Type I Crab. Superimposes X-rays and optical

May have been observed by Chaco Indians in New Mexico

Ron Lussier

1181 Type II. Now seen as radio source 3C58. THis is in X-rays

Credit: P. Slane (Harvard-Smithsonian CfA) et al., CXC, NASA

1572 Type I Tycho Gas is still very hot, so produces X-rays,seen in blue at front of blast wave

Credit: SAO, CXC, NASA

1604 Type I Kepler. Temps still in excess of 1000000°C

Kepler's SNR from Chandra, Hubble, and Spitzer Credit: R. Sankrit and W. Blair (JHU) et al., ESA, NASA Graphic: courtesy STScI

1667 Type II Cas.A

Credit: U. Hwang (GSFC/UMD), J.M. Lamming (NRL), et al., CXC, NASA,

2 Kinds, distinguished by light curves TYPE I Decay rapidly for 30 days, exponentially afterwards In all galaxies TYPE II Rapid decay -> Plateau->Rapid decay

Type 1 have a compact object (white dwarf) with a red giant, which expands and spills material onto companion, finally triggering catastrophic collapse. All type 1a seem to be the same (very important for later on!) Material ⇒ Accretion disk onto compact object Triggers explosive burning ⇒ shock wave compresses accretion disk ⇒ more burning ⇒ massive ejection of disk material ⇒ formation of 54Fe (half-life 70 days!) decay provides energy for slow light curve

Drawing Credit: ST ScI, NASA

SN2005

Supernova 2005cs in M51 (see center of right image). Blue supergiant, type II. Discovered by Wolfgang Kloehr, June 28, 2005

Many in external galaxies:spectrum show ejected material has v - 10⁴ km s^-1

Supernova Sn 1987a

Photographically February 23rd (unit is fraction of day!)

1. 23rd .042 - .055, not seen.
2. 23rd 059 - .101, not seen
3. 23rd .39, <7m
4. 23rd .443, m = 6.36
5. 23rd .62, m = 6.11
6. 24th Observed visually Sheldon
   

v. fast initial rise, then increase to plateau

3 hours before the light arrived a pulse of neutrinos hit the various detectors running at the time (Kamiokande, Mt.. Blanc) Theoretically predicted but never seen before or since.

Star could be identified with known one in catalog Sk-69°202 in Lesser Magellanic Cloud (first time we have been able to do this!) Distance ∼ 156000 lys ∼ 50 kpc ⇒ Mv = -16.0

Progenitor was blue(!) supergiant M ∼ 20M₀ May have companion star but definitely type II. Surrounded by rings before explosion

• Can now see blast wave from explosion hitting rings of material previously ejected

  Credit: P. Challis, R. Kirshner (CfA), and B. Sugerman (STScI), NASA

• Eta Carinae blew off a lot of material 150 years ago: probably pre-collapse now

  Credit: J. Morse (U. Colorado), K. Davidson (U. Minnesota) et al., WFPC2, HST, NASA

• The Crescent Nebula is s shell of gas surrounding a central Wolf-Rayet star WR 136 (very hot and unstable). Should undergo a supernova explosion in next million years.

  Credit: Brian D. Moore (ASU) et al., WFPC2, HST, NASA

• NGC 3603: can see formation of stars and probably pre-collapse Sher 25 surrounded by rings

  Credit: Wolfgang Brandner (JPL/IPAC), Eva K. Grebel (U. Wash.), You-Hua Chu (UIUC), NASA

Compact Objects

Means......

1. White Dwarfs
2. Neutron Stars
3. Black Holes
4. SS433

Mass is the critical factor for "normal" stars: the other important parameter is density

• Water (by definition) ρ = 1
• Lead ρ ∼ 13
• Osmium ρ ∼ 22
• Air ρ ∼ .001
• Earth ρ ∼ 6
• Jupiter ρ ∼ 1
• Saturn ρ ∼ .7 (yes, it would float!)
• Sun ρ ∼ 1 (but about 150 at centre, much less at outside)

(these are old units: if you compare to a modern book, multiply all densities by 1000)

White dwarfs

As seen in planetary nebula: star with about the same mass as sun but size of earth (∼10000 km )

Density: ∼ 10⁶: ∼ 100,000 times as dense as lead.

This shows some in M4 (a rich globular cluster of stars). temperature very hot: T ∼ 50000°C: since they are small, they cool very slowly.

Credit: NASA, HST, WFPC 2, H. Richer (UBC)

• "Found" by Herschel who noticed Sirius was "wobbling". Observed in 1862 by Alvan Clark: very hard to see since it is close to Sirius A but 1/10000 of the brightness, but mass ∼ 1.05 Mo. This implies a very small object: T =29500 K L ∼ 0.003 L₀. Radius= 1500 km ∼ 1/5 R of the earth. Since it is so hot, it is bright in X-rays: this shows Sirius B bright and a very dim Sirius A!

• Two oddities:
• the Dogon people of West Africa have the legend that their ancestors came from a star that circled Sirius and one day they will come back.
• Both Greek and Chinese records describe Sirius as red: it may have changed in historical times

What Sirius might have looked like: NGC 3132 (the Eight Burst Nebula), a recently formed planetary nebula with a white dwarf and companion, will probably look like Sirius in 100000 years

Credit: Hubble Heritage Team (AURA/STScI /NASA)

Neutron stars

Very regular radio pulses, period of 4 s ⇒ 2 ms Note that height of pulse is very irregular

All lie close to Milky Way (i.e. in plane of galaxy). Therefore must be related to stars

• Best known is Crab Pulsar at centre has period of ∼ .03 s. This shows how the X-ray pulses move through the nebula

Optical pulsing observed by TV or strobe
Pulses at all wavelengths, in synch.

Do we see all the pulsars?

No, because they would have to be oriented so that they point towards us. rotation period will slow down...

1. Neutron Star forms from supernova, P ∼ 1 ms
2. P ∼ 30 ms after 1000 years
   
3. P ∼ 5 s over 10⁵ years, also magnetic field may weaken, at which stage radio signal is too weak to be seen

• Hence probably large number of radio-quiet neutron stars: essentially impossible to see. Since neutron stars are so hot we see them in X-rays and γ-rays. This shows how a new satellite (GLAST) will see the sky: the brightest object is the Crab and the second brightest....... Geminga: a pulsar that had only been seen in γ-rays until it was identified as a very faint star

  GLAST Gamma Ray Sky Simulation Credit: S. Digel (USRA/ LHEA/ GSFC), NASA

Black Holes

• Einstein
• Hawking?
• Well, actually John Michell rector of Thornhill Church in Yorkshire and
  • geologist?
  • philosopher?
  • astronomer?
  • Seismologist?
  • Polymath?
• presented his ideas to the Royal Society in London in 1783. and astronomer Pierre Laplace, in 1795.

Escape Velocity

G = 6.67x10^-11 is Newton's constant, M is the mass of the object from which you are launching, so for the earth is 6x10²⁴ kg and m is the mass of the object. Note P. E. = 0 at r=∞ . The kinetic energy is Total energy is conserved, so at Earth's surface

Black Holes

• At the earth's surface
  \color{red}{ v = \sqrt {\frac{{2GM}}{R}} }
• However we can interpret this differently: what radius would the earth have for a given escape velocity?
  \color{red}{ R = \frac{{2GM}}{{v^2 }}}
• In particular, if the escape velocity is the speed of light c, nothing can escape
  \color{red}{ R = \frac{{2GM}}{{c^2 }}}
• This is the Schwarzchild radius (loosely the black-hole radius) for any mass.
• What is this for the earth?

  Statutory Warning:This is a fudge: you cannot treat light as a massive particle, nor can you handle a very strong gravitational field as if it were a weak one......

  (there are actually two factors of 2 error which cancel out.....weren't we lucky!)

But black holes are black:

as the bumper sticker says. So how do we see them?

If we are really lucky....(or unlucky) as a gap in the sky

Too Close to a Black Hole Credit & Copyright: Robert Nemiroff (MTU)

But more likely via the "accretion disk" which will have velocity ∼ c at inner edge, so temp well into X-rays

So want binary, with invisible heavy companion with M > 3M₀, emitting X-rays

Prime candidate is Cygnus X-1, which agrees with position of a massive blue star HD 226868

• Mass of primary star ∼ 20M₀
  
• Mass of invisible object M∼ 9M₀
  
• puts out 4x10³⁰ W in X-rays (i.e. 10⁴ L₀!!)

SS-433

• SS-433 found as star with very unusual spectrum X-ray source discovered at same position Spectrum changes with 164 day cycle, corresponding to v ∼ 50000 km/s

• This is something new! Narrow jets travelling at 1/5 speed of light are shot out of poles, probably formed by thick accretion disk around neutron star or black hole: "cosmic lawn sprinkler"

• And now we want to move on to a larger scale and talk about galaxies