Hi, it's Michael here.In this video we will offer you a "Guinness Book
of Records" of the most extreme stars currently known in our universe: we will
make a review of stars that for some of their peculiarities can be considered record stars.
Let's start with the fastest star. But how fast? And with respect to what? The stars are not still,
but they move around the center of our galaxy; the Sun also moves around the galactic center,
in around 200 million years. This implies that, over the course of a human life,
the stars remain more or less in the same position with respect to how our grandparents knew them;
but over millions of years, the stars that we will see at night from our planet will change
their position with respect to how we know them today, and therefore the constellations that
our very distant descendants will observe will be completely different from the current ones.
To identify the movement of a star in the sky, two types of velocity are defined:
the radial velocity, which indicates how much a star approaches or goes away from the Earth,
and the proper motion, which instead is the displacement of a star measured on the celestial
sphere, and is the parameter used to determine how fast a star is. The record belongs to Barnard's
star, which moves almost exactly towards North at a speed of 10.4 arcseconds per year.
Just to give an idea of the speed of its motion, we can also say that it moves the width of the
full moon every 200 years. Barnard's star was discovered in 1916 by the US astronomer Edward
Emerson Barnard, hence the name; it is a star of magnitude 9.5 observable in the direction of the
constellation of Ophiuchus, 4 degrees West of the bright star Beta Ophiuchi. Due to its magnitude,
it is not visible to the unaided eye, but can be easily appreciated through a
small telescope. Barnard's star is a red dwarf 5.96 light years away from Earth,
and is the second closest star to the Sun. In 2018, an exoplanet of 3.2 times the Earth's mass
was discovered orbiting Barnard's star. How do you imagine this planet? Write it in the comments!
If the second closest star to the Sun is Barnard's star, the closest to the Sun is Proxima Centauri,
which is just 4.3 light years away: it means that, if we could travel there aboard a spaceship of
science fiction movies at the speed of light, it would take 4.3 years to arrive: this means
that the light of Proxima Centauri that we see at a certain moment is not that of that moment there,
but it left 4.3 years earlier! Proxima Centauri is a red dwarf with a mass equal to one eighth
of the solar mass and a diameter equivalent to one seventh of the solar diameter but it is very
active, because it belongs to the category of flare stars: stars subject to sudden and violent
increases in brightness. These stars, unlike the larger ones like the Sun, are convective, that is,
over time all the star's hydrogen can descend towards the nucleus and be converted into helium.
As for the brightest star it is important to specify whether we are referring to the
brightest star that we can observe from Earth or to the star with the highest intrinsic brightness.
Magnitude is used to measure the brightness of a star: the apparent magnitude is the measure of the
brightness of the star observed from Earth. The absolute magnitude, on the other hand,
is connected to the intrinsic brightness of the star, and by convention it is the
magnitude the star would have if it were placed at a distance of 10 parsecs from the observer;
remember that a parsec is equivalent to about 3.26 light years.
For the first case, the brightest star we can observe from Earth is Sirius,
in the constellation of Canis Maior, at a distance of about 8.6 light years:
its apparent magnitude is -1.44! Recall that the brighter the star, the lower its magnitude,
while the less bright stars have the higher magnitudes. Sirius is a main sequence star
belonging to the spectral type A1, and its surface temperature is around 9900 K (about
4200 K higher than that of the Sun). It is about twice the diameter and mass of solar analogs.
The star with the highest intrinsic brightness, therefore with the lowest absolute magnitude known
to date is R136a1, which is located within the Tarantula Nebula in the Large Magellanic Cloud:
its absolute magnitude is -12,235! In other words, it shines like 6.2 million Suns,
has a mass equal to 215 times the solar mass and a photospheric temperature of about 46000
K! To give you an idea of how incredible this temperature is,
think that it is about a thousand times higher than the temperature in the Death Valley! If
you were outdoors on a hypothetical planet orbiting this star, probably even those of you
who are a staunch supporter of suntan wouldn’t be able to resist for more than a few seconds,
given the very strong light coming from this star. R136a1 is a Wolf-Rayet star, a particular category
of stars characterized by powerful stellar winds that make them lose their mass quickly.
Which is the faintest star instead? Today it is thought to be Van Biesbroeck 10, in short VB 10,
discovered in 1944 by George van Biesbroeck, a Belgian naturalized American astronomer:
the mass of this star is equal to just 8% the mass of the Sun, and is just enough to
allow it to produce energy through thermonuclear fusion reactions. Its absolute magnitude is 18.7,
while its visual apparent magnitude is 17.3: it would take more than 350,000 of these stars
to match the brightness of the Sun! If it were in the place of the Sun, the daylight we could enjoy
on Earth would be barely more than that offered by the full moon! What do you think it would be
like to live on Earth if Van Biesbroeck were in the place of the Sun? Write it in the comments!
The hottest star known to date is WR 102, a Wolf star in the constellation of Sagittarius,
belonging to the spectral class WO2: its photospheric temperature is approximately 210000
K! The brightness instead is around 380000 times the brightness of the Sun,
the mass is about 19 times that of the Sun while the radius is half that of our star. Its distance
is around 9400 light years. WR 102 is believed to explode as a supernova within 1500 years.
In addition to the record of the weakest star, VB 10 holds another record: that of the coldest star,
its surface temperature is just 2600 K! The filament of the classic tungsten bulb, the
metal that melts at the highest temperature, that is 3700 K, would easily resist these temperatures.
If we still go down with the temperatures, we leave the realm of the real stars,
and we enter that of the brown dwarfs, the missing stars: missed because they formed
like normal stars, but have insufficient mass to trigger the fusion of the hydrogen;
however, they fuse deuterium and possibly lithium for at least a period of their life.
In these cases, the surface temperature does not exceed 2300 K in any evolutionary phase.
To search for the oldest star we must determine its metallicity,
that is the mass fraction of the chemical elements heavier than hydrogen and helium,
which in astronomy are all considered metals. Since heavier chemical elements have formed
inside stellar nuclei through fusion reactions or supernova explosions, younger stars will have a
higher metal content than older stars, i.e. their metallicity will be higher. There are several
candidates with low metallicity, which therefore can aspire to the record for the oldest star:
- Cayrel's star, far away 4 kiloparsecs from Earth and located in the galactic halo: its estimated
age is 12.5 billion years, and this estimate was made by the French astronomer Roger Cayrel
studying the thorium and uranium content inside the star. It is located within the constellation
of Cetus and is of magnitude 11, well below the threshold of visibility to the unaided eye.
- HD140283: of magnitude 7.2, therefore within the range of binoculars, it is located in the northern
part of the constellation of Libra. Nicknamed "the star of Methuselah", it would have formed
at most 300 million years after the Big Bang. - Caffau star: named in honor of the Italian
astronomer Elisabetta Caffau who discovered it, this star has one of the lowest known
metallicities: just 1/2000 that of the Sun! It is likely to be over 13 billion years old.
As for the oldest star, for the youngest it is difficult to identify a single candidate too, also
because often the newly formed stars are still wrapped inside their gas and dust nebula, so they
may not be observable in the visible wavelengths; however we can identify a couple of them:
- T Tauri: it is a variable star within the constellation of Taurus, with an estimated age
of 1 million years, and it is so young that it is still in the pre-main sequence phase! This means
that in its core the suitable temperatures have not yet been reached to start the thermonuclear
fusion of hydrogen. T Tauri is part of a triple system together with two other protostars,
which are in an evolutionary phase even before that in which T Tauri is found. The
star T Tauri is the prototype of a homonymous type of star with similar characteristics. If
you have a telescope you can try to locate it. - FU Orionis: within the constellation of Orion,
it is a pre-main sequence star too, and is characterized by strong variations in
brightness: in 1937, suddenly, it went from magnitude 16.5 to 9.6,
with a consequent variation in brightness of 600 times! FU Orionis is the prototype of a
class of stars surrounded by a disc of matter that emit intensely in infrared and X-rays.
Let's now move on to our next record and that is to the biggest star, and even in this case
there are at least a couple of candidates who play it: Stephenson 2-18 and VY Canis Maioris.
Stephenson 2-18 is a red supergiant in the constellation of the Scutum;
is located at a distance of about 18,900 light years near the open star cluster Stephenson 2
and its brightness is 437,000 times that of the Sun: think that its radius is equal to that of
2150 times the solar radius, and consequently the its volume would be equivalent to that of
about 10 billion times the volume of our star! If this star were in the place of the Sun, it
would easily occupy the orbit of Saturn: in your opinion how big would it appear to us from Earth?
Write it in the comments! In addition to Stephenson 2-18,
another gigantic star is VY Canis Maioris, whose radius stops at "only" 1420 solar rays:
if we could fly over this star traveling at 900 km / h aboard an airliner,
it would take almost 800 years to make just one turn of the star on the equator! Mass and
luminosity of VY Canis Maioris are respectively 17 times and 270,000 times the values of the Sun.
We conclude our review of stellar records with the smallest star known today: EBLM J0555-57Ab,
which has a mass equal to just 85 times that of Jupiter and a diameter just greater than
that of the planet Saturn, just the minimum size to be able to be considered a star and
develop thermonuclear fusion reactions: in fact, if it had a slightly smaller mass,
it would have become a brown dwarf and would have cooled over time until it went out.