General Astronomy

Extreme Stars&

Other Starlike Curiosities

A New View of the Universe

The birth of Radio AstronomyIn 1928, Bell Labs wanted to investigate using "short

waves" for transatlantic radio telephone service.   Karl Jansky was assigned the job of investigating the sources of static that might interfere with radio voice transmissions.  

Jansky built an antenna designed to receive radio waves at a wavelength about 14.5 meters.  

It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round".

 

A New View of the Universe

• After recording signals from all directions for several months, Jansky identified three types of static: 1. nearby thunderstorms, 2. distant thunderstorms3. a faint steady hiss of unknown origin.  

• Jansky spent over a year investigating the third type of static.  It rose and fell once a day, leading Jansky to think at first that he was seeing radiation from the Sun.  

• But after a few months of following the signal, the brightest point moved away from the position of the Sun.  The signal repeated not every 24 hours, but every 23 hours and 56 minutes.  This is characteristic of the fixed stars, and other objects far from our solar system.  He eventually figured out that the radiation was coming from the Milky Way and was strongest in the direction of the center of our Milky Way galaxy, in the constellation of Sagittarius.

The birth of Radio Astronomy

A Starlike Curiosity

Radio astronomy got going during the 1940s and 50s, and although in its early days position accuracy was poor, by the early 1960s a number of radio sources had been identified with apparently stellar (point-like) sources.

These were labeled as QSS's:

Quasi-Stellar Sources

Quasi-Stellar SourcesThe assumption was that they were

stars, and yet their optical spectra were unlike those of any star previously observed. While resembling stars, these objects were clearly not star – just star-like, or quasi-stellar.

They produced emission lines in their spectra, but these lines did not match any elements seen in the lab.

Quasi-Stellar ObjectsIn 1963, Martin Schmidt looking at the rather

dim spectra of an object known as 3C273 and realized that the unknown elements in the emission spectra were rather common – Hydrogen, but were redshifted 15.8%

A 15.8% redshift corresponds to a distance of about 2 billion lightyears

• It was not the 15% redshift that puzzled Schmidt, galaxies were already known with much larger redshifts, but rather the brightness of 3C273.

• 3C273 was a thousand times brighter than even a very luminous galaxy would appear at a distance of 2 billion light years

Quasi-Stellar Objects

Quasars• Once identified as an actual entity, they were

renamed Quasi-Stellar OBJECTS• QSO's are now known as Quasars• Soon even higher redshift quasars were discovered

- there has been a longstanding tradition that the discoverer of the highest redshift quasar is awarded a case of champagne.

• The current record Quasar has a redshift, z = 5.5 determined by astronomers at JPL and elsewhere

Current high redshift• The z = 5.5 (i.e., redshifted

550%) quasar is the center red object

• A redshift of 5.5 corresponds to a velocity of 95% of the speed of light, or a distance approaching 14 billion light-years.

Twinkle, twinkle, quasi-star,Biggest puzzle from afar.

How unlike the other ones,Brighter than a trillion

Suns.Twinkle, twinkle, quasi-

star, How I wonder what you

are!

- George Gamow

QuasarsSchmidt's realization of the redshifted nature of

quasars immediately put them far outside our Galaxy, and in fact far outside the local group of galaxies. At the same time it implied vast luminosities. The brightest quasar has a luminosity 100 times larger than that of a large galaxy. Luminosity1015 L

The minimum for Quasar is 1011 L

SpectrumBroad AND Narrow Emission LinesSome are radio-quiet, some radio-loudHigh X-ray emission 3C273

Quasar Variability

More remarkably still, the brightness of quasars can change in a matter of weeks.

This is extraordinary because we don't expect astrophysical objects to be able to change more quickly than the time taken for light to travel across them.

QuasarsEven if all the stars in M31 went out at once, we would

actually see those closest to us go out first, and it would be tens of thousands of years before the most distant followed suit.

Therefore the brightness changes of quasars implies that their huge luminosity comes from a region of less than 0.01 pc in size!

The combination of vast luminosity and small size allows them to outshine their host galaxies and appear stellar, although in recent years some host galaxies of quasars have been imaged, most notably with the Hubble Space Telescope because good resolution is essential.

Quasar Size

2Ly

1 Ly

2Ly

1 Ly

The variability of 3C273 and other quasars requires that the quasar produce its luminosity (greater than a thousand galaxies of billions of stars) from a region smaller than our solar system!

The Extreme StarsLet's take a look at some of

the (currently known) extreme cases:

The LargestThe SmallestThe Coolest

The HottestThe BrightestThe Dimmest

The Closest

The Smallest StarsCertainly, the smallest are

the neutron stars, but if we restrict ourselves to a more standard star, we have to look at the lower-left end of the HR diagram.

The smallest and hottest stars are the white dwarfs.

At about ¾ of the size of the Earth, these stars can reach a temperature of 250,000 K White Dwarves in M4

The Biggest Stars

BetelgeuseAntares

Mu Cephei

V V Cephei

Luminous Blue Variables• Luminous Blue Variable stars (LBVs) are thought to

represent an evolutionary phase in the lives of massive stars in between the main sequence and the Wolf-Rayet stage. The LBV phase will last only some 10,000 to 100,000 yr.

• Approximately 30 LBVs are known.• Giant eruptions have been observed in Eta Car and

P Cygni.• At minimum visual brightness, the star is of late-O or

B spectral type, while at maximum visual brightness the star is of spectral type mid-A to F.

Eta Carinae

LBV 1806-20

University of Florida/Meghan Kennedy

One of the biggest and brightest stars in the Milkyway: This one, discovered by Steven Eikenberry of the University of Florida, weighs in at about 150 solar masses and is about 4 million times brighter than the Sun.

It is about 45,000 Ly away.

HD 56925This is one of the rare

Wolf-Rayet stars, whose intrinsic brilliance is combined with high rates of mass loss from its surface.

This material is ejected from the star with velocities which approach 2000 Km/sec.

This violent activity quickly reduces the mass of the star which in turn reduces the instability.

The rapidly-moving ejected material interacts with the gas and dust around the star producing the cosmic bubble seen here.

The Brightest Star• The brightest star doesn't even have a

name, just a catalog designation, HD93129A

• An obscure 7th magnitude, O3 star in Carina, it takes the prize as the most luminous star. While it falls just short of being a visible star to the naked eye, its distance is 11,200 lightyears

• In absolute magnitude, it has a value of –12. That makes it 5 million times brighter than the Sun.

Carbon Stars• Carbon Stars are giant

stars with a large abundance of carbon left over from their years of nuclear fusion. Almost always these stars can be seen quite easily in a star field by their sharp red color.

• The cause for this reddening is that the star's outside layers contain quantities of the carbon molecules C2, CN and CH which creates an absorption spectrum that blocks out most blue wavelengths from the star's interior.

The Dimmest StarsA dim double star system cataloged as

Gliese 623 lies 25 light-years from Earth, in the constellation of Hercules. The individual stars of this binary system were distinguished for the first time when the Hubble Space Telescope's Faint Object Camera recorded this image in June 1994. They are separated by 200 million miles - about twice the Earth/Sun distance. On the right, the fainter Gliese 623b is 60,000 times less luminous than the Sun and approximately 10 times less massive.

The fuzzy rings around its brighter companion, Gliese 623a, are image artifacts. The lowest mass stars are classified as red dwarf stars, but even red dwarfs are massive enough to trigger hydrogen fusion in their cores to sustain their feeble starlight. The present estimates of the mass of Gliese 623b are right at the red dwarf/brown dwarf border

Close EncountersThe small star shown in the

center of the image will one day be our closest neighbor.

The faint 9th magnitude red dwarf, currently 63 Ly away was recently discovered to be approaching our Solar System.

Known as Gliese 710 it is predicted to come within 1 light-year of the Sun 1.4 million years from now.

At that distance this star, presently much too faint to be seen by the naked eye, will blaze at 0.6 magnitude about the same as Antares