Category: universo

VV 340, pair of interacting galaxies in Boöt…

VV 340, pair of interacting galaxies in Boötes.

The two galaxies shown here are in the early stage of an interaction that will eventually lead to them merging in millions of years.

Credit:

NASA/STScI/NRAO/A.Evans et al

    

Abell 2744: Pandora’s Cluster of Galax…

Abell 2744: Pandora’s Cluster of Galaxies 

Image Credit: NASA, ESA, J. Merten (ITA, AOB), & D. Coe (STScI)

Remnants from a star that exploded thousands …

Remnants from a star that exploded thousands of years ago created a
celestial abstract portrait, as captured in this NASA Hubble Space
Telescope image of the Pencil Nebula.

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

NGC 1309: Spiral Galaxy and Friends Image Cr…

NGC 1309: Spiral Galaxy and Friends 

Image Credit: Hubble Legacy Archive, ESA, NASA; Processing – Jeff Signorelli

NGC 2736: The Pencil Nebula Image Credit: Ho…

NGC 2736: The Pencil Nebula 

Image Credit: Howard Hedlund & Dave Jurasevich, Las Campanas Obs.

What is a protostar?The formation of stars beg…

What is a protostar?

The formation of stars begins with the collapse and fragmentation of molecular clouds into very dense clumps.  These clumps initially contain ~0.01 solar masses of material, but increase in mass as surrounding material is accumulated through accretion.  The temperature of the material also increases while the area
over which it is spread decreases as gravitational contraction
continues, forming a more stellar-like object in the process.  During
this time, and up until hydrogen burning begins and it joins the main sequence, the object is known as a protostar.

This stage of stellar evolution may last for between 100,000 and 10 million years depending on the size of the star being formed. If the final result is a protostar with more than 0.08 solar
masses, it will go on to begin hydrogen burning and will join the main
sequence as a normal star.  For protostars with masses less than this,
temperatures are not sufficient for hydrogen burning to begin and they
become brown dwarf stars.

Protostars are enshrouded in gas and dust and are not detectable at visible wavelengths.  To study this very early stage of stellar evolution, astronomers must use infrared or microwave wavelengths.

Protostars are also known as Young Stellar Objects (YSOs).

Red dwarf stars

Our Sun is such a familiar sight in the sky that you might think stars
like our Sun are common across the Universe. But the most common stars
in the Universe are actually much smaller and less massive than the Sun.
The Universe is filled with red dwarf stars.

Astronomers categorize a red dwarf as any star less than half the mass of the Sun, down to about 7.5% the mass of the Sun. Red dwarfs
can’t get less massive than 0.075 times the mass of the Sun because
then they’d be too small to sustain nuclear fusion in their cores.

Red dwarfs do everything at a slower rate. Since they’re a fraction of
the mass of the Sun, red dwarfs generate as little as 1/10,000th the
energy of the Sun. This means they consume their stores of hydrogen fuel
at a fraction of the rate that a star like the Sun goes through. The
largest known red dwarf has only 10% the luminosity of the Sun.

And red dwarfs have another advantage. Larger stars, like the Sun, have a
core, surrounded by a radiative zone, surrounded by a convective zone.
Energy can only pass from the core through the radiative zone by
emission and absorption by particles in the zone. A single photon can
take more than 100,000 years to make this journey. Outside the radiative
zone is a star’s convective zone. In this region, columns of hot plasma
carry the heat from the radiative zone up to the surface of the star.

Red dwarfs have no radiative zone, which means that the convective
zone comes right down to the star’s core and carries away heat. It also
mixes up the hydrogen fuel and carries away the helium by-product.
Regular stars die when they use up just the hydrogen in their cores,
while red dwarfs keep all their hydrogen mixed up and will only die when
they’ve used up every last drop.

With such an efficient use of hydrogen, red dwarf stars with 10% the
mass of the Sun are through to live 10 trillion years. Our own Sun will
only last about 12 billion or so.

Many red dwarfs are orbited by exoplanets, but large Jupiter-sized
planets are comparatively rare. Doppler surveys of a wide variety of
stars indicate about 1 in 6 stars with twice the mass of the Sun are
orbited by one or more Jupiter-sized planets, versus 1 in 16 for
Sun-like stars and only 1 in 50 for red dwarfs.

Habitability

Planetary habitability
of red dwarf systems is subject to some debate. In spite of their great
numbers and long lifespans, there are several factors which may make
life difficult on planets around a red dwarf. First, planets in the
habitable zone of a red dwarf would be so close to the parent star that
they would likely be tidally locked.
This would mean that one side would be in perpetual daylight and the
other in eternal night. 

This could create enormous temperature
variations from one side of the planet to the other. Such conditions
would appear to make it difficult for forms of life similar to those on
Earth to evolve. And it appears there is a great problem with the
atmosphere of such tidally locked planets: the perpetual night zone
would be cold enough to freeze the main gases of their atmospheres,
leaving the daylight zone bare and dry. On the other hand, recent
theories propose that either a thick atmosphere or planetary ocean could
potentially circulate heat around such a planet.

Variability in stellar energy output may also have negative impacts on the development of life. Red dwarfs are often flare stars,
which can emit gigantic flares, doubling their brightness in minutes.

This variability may also make it difficult for life to develop and
persist near a red dwarf. It may be possible for a planet orbiting close
to a red dwarf to keep its atmosphere even if the star flares. However, more-recent research suggests that these stars may be the
source of constant high-energy flares and very large magnetic fields,
diminishing the possibility of life as we know it. Whether this is a
peculiarity of the star under examination or a feature of the entire
class remains to be determined. (source) (source)

Galaxy M74 (via apod)

Galaxy M74 (via apod)

Merope Nebula in Plaiades by Karol Masztaler…

Merope Nebula in Plaiades by Karol Masztalerz 

Picture of NGC 7635 captured in narrowband b…

Picture of NGC 7635 captured in narrowband by amateur astronomer Luca Moretti