Transcript General Astronomy - Stockton University

General Astronomy
Extreme Stars
&
Other Starlike Curiosities
A New View of the Universe
The birth of Radio Astronomy
In 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.
A New View of the Universe
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".
The birth of Radio Astronomy
•
After recording signals from all directions for several months, Jansky
identified three types of static:
1. nearby thunderstorms,
2. distant thunderstorms
3. 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.
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 Sources
The 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 Objects
In 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
Quasi-Stellar Objects
• 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
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
Quasars
Schmidt'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.
Luminosity
1015 L
The minimum for Quasar is 1011 L
Spectrum
Broad AND Narrow Emission Lines
Some are radio-quiet, some radioloud
High 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.
Quasars
Even 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
2Ly
1 Ly
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 Stars
Let's take a look at some of the
(currently known) extreme cases:
The Largest
The Smallest
The Coolest
The Hottest
The Brightest
The Dimmest
The Closest
The Smallest Stars
Certainly, the smallest are
the neutron stars, but if
we restrict ourselves to a
more standard star, we
have to look at the lowerleft 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
Betelgeuse
Antares
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
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.
University of Florida/Meghan Kennedy
HD 56925
This is one of the rare WolfRayet 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 Stars
A 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 Encounters
The 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