Showing posts with label astrophysics. Show all posts
Showing posts with label astrophysics. Show all posts

Friday 24 May 2013

A liquid telescope

http://en.wikipedia.org/wiki/Liquid_mirror_telescope

Liquid mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury. The container for the liquid is rotating so that the liquid assumes a paraboloidal shape. A paraboloidal shape is precisely the shape needed for the primary mirror of a telescope. The rotating liquid assumes the paraboloidal shape regardless of the container's shape.  Liquid mirrors can be a low cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid metal mirror is much less expensive to manufacture.

"Isaac Newton noted that the free surface of a rotating liquid forms a circular paraboloid and can therefore be used as a telescope, but he could not actually build one because he had no way to stabilize the speed of rotation[citation needed] (the electric motor did not exist yet). The concept was further developed by Ernesto Capocci of the Naples Observatory (1850), but it was not until 1872 that Henry Skey of Dunedin, New Zealand constructed the first working laboratory liquid mirror telescope."

"Another difficulty is that a telescope with a liquid metal mirror can only be used in zenith telescopes that look straight up at the zenith, so it is not suitable for investigations where the telescope must remain pointing at the same location of space ... Currently, the mercury mirror of the Large Zenith Telescope in Canada is the largest liquid metal mirror in operation. It has a diameter of six meters, and rotates at a rate of about 8.5 revolutions per minute."

Adiabatica


Cosmic Microwave Background Radiation
http://www.nicadd.niu.edu/~bterzic/PHYS652/Lecture_19.pdf  by Balša Terzić  
"The CMB radiation is a prediction of Big Bang theory. According to the Big Bang theory, the
early Universe was made up of a hot plasma of photons, electrons and baryons. The photons were
constantly interacting with the plasma through Thomson scattering. As the Universe expanded,
adiabatic cooling caused the plasma to cool until it became favorable for electrons to combine
with protons and form hydrogen atoms. This happened at around 3,000 K or when the Universe
was approximately 380,000 years old (z ≈ 1100). At this point, the photons scattered off the
now neutral atoms and began to travel freely through space. This process is called recombination
or decoupling (referring to electrons combining with nuclei and to the decoupling of matter and
radiation respectively). The photons have continued cooling ever since; they have now reached 2.725 K and their temperature will continue to drop as long as the Universe continues expanding. Accordingly,
the radiation from the sky that we measure today comes from a spherical surface, called the surface of last scattering. This represents the collection of points in space (currently around 46 billion light years from the Earth) at which the decoupling event happened long enough ago (less than 400,000 years after the Big Bang, 13.7 billion years ago) that the light from that part of space is just reaching observers."

Thursday 16 May 2013

Deep Space Beacon


Pulsed gamma rays from the Vela pulsar  from photons detected by Fermi's Large Area Telescope. The Vela pulsar is the brightest persistent source of gamma rays in the sky. The bluer colour in the latter part of the pulse indicates the presence of gamma rays with energies exceeding a billion electron volts (1 GeV). For comparison, visible light has energies between two and three electron volts. Red indicates gamma rays with energies less than 300 million electron volts (MeV); green, gamma rays between 300 MeV and 1 GeV; and blue shows gamma rays greater than 1 GeV. The image frame is 30 degrees across. The background, which shows diffuse gamma-ray emission from the Milky Way, is about 15 times brighter here than it actually is.
Source Goddard Space Flight Center
Author Roger Romani (Stanford University) (Lead), Lucas Guillemot (CENBG), Francis Reddy (SPSYS)

Friday 1 June 2012

Transit of Mercury

Very beautiful image at http://apod.nasa.gov/apod/ap120527.html Image Credit: SOHO - EIT Consortium, NASA "The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk in 2003 ... the horizon was certainly no problemfor the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface."

Here the image from NASA after processing with IRIS


Transit of Venus

"The next transit of Venus, where Venus appears as a dark spot in front of the Sun, will begin at 22:09 UTC on 5 June 2012, and will finish at 04:49 UTC on 6 June.[1] Depending on the position of the observer, the exact times can vary by up to ±7 minutes. Transits of Venus occur in pairs that are eight years apart: the previous transit was in June 2004, and the next pair of transits will occur in December 2117 and December 2125." from Wikipedia

Aristarchus proposed to measure the distance to the Sun using parallax. This approach based on the geometric principles of parallax last for two thousands of years, until Edmond Halley in 1716 proposed to observe the transit of Venus. The use of Venus transits gave an estimate of 1.53×10^13 cm, 2.6% above the currently accepted value, that of l.49 × 10^13 cm. More recently, in 1910, the parallax was measured using the asteroid Eros that passed much closer to Earth than Venus. A transit of Venus happens when this planet passes directly between the Sun and Earth, appearing as a small black disk moving across the Sun bright disk. The duration of such transits is usually measured in hours.
Read  more "Two amateur astronomers at Berkeley", at http://arxiv.org/abs/1202.0950

Thursday 1 March 2012

More planets than stars

Microlensing suggests that our galaxy has more planets than stars, buBertram M. Schwarzschild
March 2012, http://dx.doi.org/10.1063/PT.3.1463
Gravitational bending of light reveals exoplanets with large orbital radii.
"Most of the more than 600 exoplanets discovered to date have been found through Doppler evidence of periodic host-star motion or photometric evidence of transits across a star’s face. Both methods are strongly biased in favor of planets with orbital radii much smaller than Earth’s, which defines 1 astronomical unit (AU). Gravitational microlensing is an alternative technique that’s most sensitive to planets a few AU from their stars. It favors very distant stars and it’s relatively unbiased as to stellar mass. Though microlensing’s discovery rate is still modest, it appeals to those who seek a representative galactic survey of planets with orbits like those of the solar system."  http://www.physicstoday.org/resource/1/phtoad/v65/i3/p19_s1

Friday 30 September 2011

Speed of neutrino and cosmic consequences

Let me report a part of the discussion on the speed of neutrinos by Wikipedia
"In September 2011 the OPERA collaboration released calculations showing velocities of 17-GeV and 28-GeV neutrinos exceeding the speed of light in their experiments. The authors write, "Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly." This result had not been detected by previous experiments, and lies in contrast to several others. For instance, photons and neutrinos from SN 1987A were observed to have an agreement in transit time to about 1 part in 450 million, with even this difference being accounted for by light being impeded by the material of the star early in its journey. The OPERA results, in contrast, suggested that neutrinos were traveling faster than light by a factor of 1 in 40,000, i.e. that neutrino speed is 1.0000248(28) c. Had neutrinos from SN 1987A (a supernova, approximately 168,000 light-years from Earth, http://en.wikipedia.org/wiki/SN_1987A) traveled faster than light by this factor, they would have arrived at Earth several years before the photons; this was not observed to be the case. However, neutrinos from the supernova had orders of magnitude less energy than the neutrinos observed in the OPERA experiment, as the authors point out."

Friday 1 July 2011

A far quasar

"A team of European astronomers, including UK astronomers, have discovered a bright quasar that has been beaming light since the Universe was a mere 770 million years old.
The brilliant beacon, named ULAS J1120+0641, is powered by a black hole with a mass two billion times that of the Sun. Located at a redshift – a term relating to astronomical distances – of 7.1, its light has taken 12.9 billion years to reach us. The next most distant quasar is seen at 870 million years after the big bang, or a redshift of 6.4, although gamma ray bursts have been detected at greater distances of 8.6 and 8.2 redshifts."
Most distant quasar shines brightly

Friday 24 June 2011

Voyager mission at the edge of the solar system

"Recent data from the spacecraft have shown a gentle decrease in the velocity of the solar wind at the heliopause – the outer boundary of the heliosheath – not the abrupt discontinuity predicted by current theories. Also, scientists looking at other data from both Voyager 1 and Voyager 2 have found that the magnetic field in the heliosheath is a tumultuous foam of magnetic bubbles, as compared to the graceful arcs of magnetic field lines they had expected."
More surprises for the Voyager mission at the edge of the solar system - physicsworld.com

Wednesday 25 May 2011

Telescope optics set to aid gravitational detection

"A British team is designing the optics for a telescope that will be able to detect the gravitational effects of violent cosmic events, such as when two black holes collide.
The €790m (£688m) Einstein Telescope should be completed by 2025, by which time it will be capable of detecting gravitational waves around 100 orders of magnitude fainter than current devices can." Telescope optics set to aid gravitational detection News The Engineer

Friday 20 May 2011

Unbound planets could abound in the universe

"Ten planets that appear to be drifting in interstellar space have been spotted by an international team of astronomers. The planets are so far from any host stars that they may not orbit a star at all, and could be drifting unbound through space. The team believes that such rogue planets could outnumber normal stars almost 2:1 and their existence could confirm computer simulations of solar-system formation."
Unbound planets could abound in the universe - physicsworld.com

Thursday 19 May 2011

Wandering planets

"The Milky Way might be filled with hundreds of billions of gas-giant planets that were ejected from the planetary systems that gave them birth and either were going their own lonely ways or were only distantly bound to stars at least 10 times as far away as the sun is from the Earth. There are two Jupiter-mass planets floating around for each of the 200 billion stars in the Milky Way galaxy, according to measurements and calculations by an international group of astronomers led by Takahiro Sumi, of Osaka University in Japan, and reported in the journal Nature."
Stunned scientists discover Milky Way awash with planets 
The Sydney Morning Herald

Monday 11 April 2011

SNEWS

When a red giant star collaps on itself a burst of neutrinos is produced. This occurs before that light is emitted in the explosion, The SuperNova Early Warning System (SNEWS) is a network of neutrino detectors designed to give an early warning to astronomers of a supernova event in the Milky Way. The neutrino pulse from supernova 1987A was detected 3 hours before the photons.
The current members of SNEWS are Borexino, Super-Kamiokande, LVD, SNO and IceCube.
http://en.wikipedia.org/wiki/Supernova_Early_Warning_System