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From Wikipedia, the free encyclopedia

Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space".[1][2] Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background.[3][4] Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

In practice, modern astronomical research often involves a substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter, dark energy, and black holes; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the origin and ultimate fate of the universe.[3] Topics also studied by theoretical astrophysicists include Solar System formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics.

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  • ✪ Astrophysics and Cosmology: Crash Course Physics #46
  • ✪ What You Should Know About Getting a Career In Astronomy/Astrophysics
  • ✪ So You Want To Get an Astronomy/Astrophysics Degree
  • ✪ This is what an astrophysics exam looks like at MIT
  • ✪ What's the Difference Between an Astronomer and an Astrophysicist?


After a year of studying the laws of the universe together, a year of studying and calculating, learning about motion and fluids, thermodynamics, electricity and magnetism, light and sound, We find ourselves here: Space, the final frontier. We’re all voyagers on a mission to understand the universe, using the power of physics. And even though we’ve been doing it for centuries, there’s so much we have to learn about the cosmos! Some of the most exciting research in physics today is being done by astrophysicists and cosmologists. Astrophysicists study the physics of celestial bodies, such as planets, stars, and galaxies. Their research takes us inside phenomena like black holes and supernovae. But we can use physics to try to answer even bigger questions about the universe. Cosmologists study the universe overall and ask questions about the origin of everything, as well as its future. It’s their job, and yours, to continue looking into the night sky, searching for answers using the tools and knowledge that physics can provide. [Theme Music] Before we can talk about something as big as the universe, we need to be able to describe just how big it is. When talking about things on Earth, we typically use measurements in the range of nanometers to kilometers. But when we’re in space, we need something a whole lot bigger. For instance, I could say that the nearest star to earth, besides the sun, is 4x10^13 kilometers from us. But that’s a mouthful for something that’s basically right next door, in cosmic terms. So it’s easier to say that this star, Proxima Centauri, is 4.2 light-years from Earth. A light-year is a unit of length, with one light-year equaling the distance that light would travel in a vacuum in one year. If you take the speed of light, about 300 million meters per second, and multiply it by how many seconds are in a year, you find that one light-year is approximately 10^16 meters, or 10 million, billion meters. To give you a sense of scale, it takes light just over 8 minutes to travel from the sun to the Earth, and the Milky Way is roughly a hundred thousand light-years in diameter. Sometimes we also use a unit called a parsec, which is equal to 3.26 light years. Now, when we say that Proxima Centauri is 4.2 light-years away, this also means that when we look at the star through a telescope, we’re seeing what Proxima Centauri looked like 4.2 years ago. It takes light that long to get here from Proxima Centauri, so we’ll never know what that star – or any star or other distant object – looks like at this exact moment. This means that, as we observe celestial objects far away, we’re looking into the past, seeing what stars and galaxies looked like millions, if not billions, of years ago. While we’re observing these stars, we can use a spectrometer, the device that separates wavelengths, to reveal the star’s absorption spectrum and its elemental composition. But when we study very distant bodies, we find that their absorption spectrum is slightly different from what we’d expect, given our knowledge of typical star compositions. Remember the Doppler effect? How the pitch of an ambulance siren becomes higher as it approaches you and lower as it moves away? The same effect happens with light! If an object is moving away from you, the speed of light doesn’t change, but the peaks of the electromagnetic wave that it emits move farther apart. This effect – which occurs with light emitted by an object moving away from Earth – is called redshift, because the longer the wavelengths get, the closer they are to the red part of the visible spectrum. Once astronomers recognized and could measure redshift, they found that the spectra from nearly every distant galaxy was redshifted, meaning that every galaxy was moving away from us. And if that wasn’t strange enough, astronomers such as Edwin Hubble noted that the amount of redshift is proportional to the distance from Earth. So the farthest galaxies are moving away even faster than the close ones! Georges Lemaître, a Belgian physicist, proposed this relationship as Hubble’s Law. It expresses the velocity at which a galaxy is speeding away from Earth, in terms of its distance from us. The equation uses a proportionality constant called the Hubble parameter, which says that for every million light-years of distance from us, a galaxy is moving away at an additional 21 kilometers per second. Now, the fact is that nearby galaxies might be going away from us or toward us, based on how they’re moving within their local cluster. But the overall tendency for distant galaxies to recede from us is much more common, so Hubble’s Law holds true in most cases. And this trend of distant galaxies moving away from us, and from one another, is called cosmological redshift. By the way, this expansion looks the same whether you’re on Earth or not. No matter where you are, all distant galaxies appear to be moving away from you. So this leads physicists to believe that at some point in time, all the stars and galaxies were closer to one another – a lot closer. In the 1940s and 50s, Russian-American physicist George Gamow, developed a theory of the early universe that explained, among other things, why so many light elements, like hydrogen and helium, were observed throughout the cosmos. He suggested that the universe began in a state of highly compressed hot plasma – a sort of hot soup of elementary particles. His theory became known, somewhat dismissively by his colleagues, as the Big Bang Theory. And this same theory ultimately predicted that there should be radiation left over from the initial, rapid expansion of that compressed plasma. This is because hot plasma, like the plasma in the flame of a candle, is not transparent. But ordinary gas – like the air around a candle’s flame – is transparent, and it lets light travel freely through it. So it would make sense that the early, hot universe was originally opaque, until it cooled down to the point where it became transparent. Once that happened, the thinking goes, light from the Big Bang was able to travel freely. But its wavelength kept stretching out, redshifting until it could only be detected as microwave radiation. Gamow's theory didn’t gain much acceptance, and it was largely forgotten. Until, in 1964, American astronomers Arno Penzias and Robert Wilson pointed a radio antenna into space, and they discovered cosmic microwave background radiation. They basically discovered the radiation from the Big Bang, by accident. They found that a low-energy microwave radiation persisted at all times, day and night, and they concluded that the source of the radiation was the universe itself. This cosmic background radiation provides support for the Big Bang Theory, and it tells us a lot about the conditions of the early universe. Thanks to these insights, along with the observed expansion of the universe and other evidence, we have learned that the universe began in a hot dense state, then cooled, and produced galaxies and clusters that we see today. However, many mysteries remain. For instance, if the universe started with such high density and temperature, wouldn’t gravity make its expansion slow down? The fact is, the rate of expansion would slow down only if the universe was filled with nothing but matter and radiation. But that’s not the case! Space is filled with a constant – or at least, slowly varying – form of energy known as dark energy. And because of the pervasive presence of this energy, according to general relativity, gravity is actually causing space to expand, and accelerate! This isn’t just theoretical. Recent evidence suggests that the universe actually IS accelerating in its expansion, showing no signs of slowing down. But beyond the fact that it exists, there’s not much that we know about dark energy. Another one of the universe’s great mysteries is the existence of mass that we can’t see, but we know that it, too, exists. When we study a galaxy’s rotation, we can estimate how much mass is in it by measuring its radius and rotational velocity. But when we actually calculate that mass, the result is far greater than what’s observable as stars and gas. The conclusion is that there’s an immense amount of mass in the universe known as dark matter, which doesn’t reflect or emit any light. By current estimates, dark matter makes up almost 85 percent of all the matter in the universe. This means that all visible matter, including stars and planets, make up just a small percentage of all energy in the universe, while the rest is mysterious dark energy and dark matter. Research in these fields is ongoing and new evidence is found every year, refining our understanding of the universe. In the past year here on Crash Course, and for thousands of years, we have used physics to answer some of life’s most important questions. Whether it’s a ball flying through the air or the origin of the universe, we can use our knowledge from Newton’s Laws to special relativity in order to move closer to the truth. And there’s still so much to discover, both in the stars and here on earth. Some of the most groundbreaking research is happening on the smallest scales, as physicists seek to understand the building blocks of our universe and the nature of matter itself. It’s not an easy task, and it’s why we need scientists and enthusiastic supporters such as yourself to go out, be curious, ask questions, and to find answers through scientific methods. Today we learned about light-years and how looking in the distance is also looking into the past. We discussed redshift and used Hubble’s Law to calculate how much certain parts of the universe are expanding away from us. Finally, we introduced the Big Bang, cosmic background radiation, and the mysteries of dark energy and dark matter. Bye! Crash Course Physics is produced in association with PBS Digital Studios. You can head over to their channel to check out a playlist of their latest amazing shows like: The Art Assignment, Gross Science, and Deep Look. This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio with the help of these amazing people and our equally amazing graphics team, is Thought Cafe.



Early 20th-century comparison of elemental, solar, and stellar spectra
Early 20th-century comparison of elemental, solar, and stellar spectra

Although astronomy is as ancient as recorded history itself, it was long separated from the study of terrestrial physics. In the Aristotelian worldview, bodies in the sky appeared to be unchanging spheres whose only motion was uniform motion in a circle, while the earthly world was the realm which underwent growth and decay and in which natural motion was in a straight line and ended when the moving object reached its goal. Consequently, it was held that the celestial region was made of a fundamentally different kind of matter from that found in the terrestrial sphere; either Fire as maintained by Plato, or Aether as maintained by Aristotle.[5][6] During the 17th century, natural philosophers such as Galileo,[7] Descartes,[8] and Newton[9] began to maintain that the celestial and terrestrial regions were made of similar kinds of material and were subject to the same natural laws.[10] Their challenge was that the tools had not yet been invented with which to prove these assertions.[11]

For much of the nineteenth century, astronomical research was focused on the routine work of measuring the positions and computing the motions of astronomical objects.[12][13] A new astronomy, soon to be called astrophysics, began to emerge when William Hyde Wollaston and Joseph von Fraunhofer independently discovered that, when decomposing the light from the Sun, a multitude of dark lines (regions where there was less or no light) were observed in the spectrum.[14] By 1860 the physicist, Gustav Kirchhoff, and the chemist, Robert Bunsen, had demonstrated that the dark lines in the solar spectrum corresponded to bright lines in the spectra of known gases, specific lines corresponding to unique chemical elements.[15] Kirchhoff deduced that the dark lines in the solar spectrum are caused by absorption by chemical elements in the Solar atmosphere.[16] In this way it was proved that the chemical elements found in the Sun and stars were also found on Earth.

Among those who extended the study of solar and stellar spectra was Norman Lockyer, who in 1868 detected bright, as well as dark, lines in solar spectra. Working with the chemist, Edward Frankland, to investigate the spectra of elements at various temperatures and pressures, he could not associate a yellow line in the solar spectrum with any known elements. He thus claimed the line represented a new element, which was called helium, after the Greek Helios, the Sun personified.[17][18]

In 1885, Edward C. Pickering undertook an ambitious program of stellar spectral classification at Harvard College Observatory, in which a team of woman computers, notably Williamina Fleming, Antonia Maury, and Annie Jump Cannon, classified the spectra recorded on photographic plates. By 1890, a catalog of over 10,000 stars had been prepared that grouped them into thirteen spectral types. Following Pickering's vision, by 1924 Cannon expanded the catalog to nine volumes and over a quarter of a million stars, developing the Harvard Classification Scheme which was accepted for worldwide use in 1922.[19]

In 1895, George Ellery Hale and James E. Keeler, along with a group of ten associate editors from Europe and the United States,[20] established The Astrophysical Journal: An International Review of Spectroscopy and Astronomical Physics.[21] It was intended that the journal would fill the gap between journals in astronomy and physics, providing a venue for publication of articles on astronomical applications of the spectroscope; on laboratory research closely allied to astronomical physics, including wavelength determinations of metallic and gaseous spectra and experiments on radiation and absorption; on theories of the Sun, Moon, planets, comets, meteors, and nebulae; and on instrumentation for telescopes and laboratories.[20]

Around 1920, following the discovery of the Hertsprung-Russell diagram still used as the basis for classifying stars and their evolution, Arthur Eddington anticipated the discovery and mechanism of nuclear fusion processes in stars, in his paper The Internal Constitution of the Stars.[22][23] At that time, the source of stellar energy was a complete mystery; Eddington correctly speculated that the source was fusion of hydrogen into helium, liberating enormous energy according to Einstein's equation E = mc2. This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed of hydrogen (see metallicity), had not yet been discovered.[non-primary source needed]

In 1925 Cecilia Helena Payne (later Cecilia Payne-Gaposchkin) wrote an influential doctoral dissertation at Radcliffe College, in which she applied ionization theory to stellar atmospheres to relate the spectral classes to the temperature of stars.[24] Most significantly, she discovered that hydrogen and helium were the principal components of stars. Despite Eddington's suggestion, this discovery was so unexpected that her dissertation readers convinced her to modify the conclusion before publication. However, later research confirmed her discovery.[25]

By the end of the 20th century, studies of astronomical spectra had expanded to cover wavelengths extending from radio waves through optical, x-ray, and gamma wavelengths.[26] In the 21st century it further expanded to include observations based on gravitational waves.

Observational astrophysics

Supernova remnant LMC N 63A imaged in x-ray (blue), optical (green) and radio (red) wavelengths. The X-ray glow is from material heated to about ten million degrees Celsius by a shock wave generated by the supernova explosion.

Observational astronomy is a division of the astronomical science that is concerned with recording data, in contrast with theoretical astrophysics, which is mainly concerned with finding out the measurable implications of physical models. It is the practice of observing celestial objects by using telescopes and other astronomical apparatus.

The majority of astrophysical observations are made using the electromagnetic spectrum.

Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances. A few gravitational wave observatories have been constructed, but gravitational waves are extremely difficult to detect. Neutrino observatories have also been built, primarily to study our Sun. Cosmic rays consisting of very high energy particles can be observed hitting the Earth's atmosphere.

Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. However, historical data on some objects is available, spanning centuries or millennia. On the other hand, radio observations may look at events on a millisecond timescale (millisecond pulsars) or combine years of data (pulsar deceleration studies). The information obtained from these different timescales is very different.

The study of our very own Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Our understanding of our own Sun serves as a guide to our understanding of other stars.

The topic of how stars change, or stellar evolution, is often modeled by placing the varieties of star types in their respective positions on the Hertzsprung–Russell diagram, which can be viewed as representing the state of a stellar object, from birth to destruction.

Theoretical astrophysics

Stream lines on this simulation of a supernova show the flow of matter behind the shock wave giving clues as to the origin of pulsars
Stream lines on this simulation of a supernova show the flow of matter behind the shock wave giving clues as to the origin of pulsars

Theoretical astrophysicists use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.[27][28]

Theorists in astrophysics endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute a model or help in choosing between several alternate or conflicting models.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.

Topics studied by theoretical astrophysicists include stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astrophysics, now included in the Lambda-CDM model, are the Big Bang, cosmic inflation, dark matter, dark energy and fundamental theories of physics. Wormholes are examples of hypotheses which are yet to be proven (or disproven).


The roots of astrophysics can be found in the seventeenth century emergence of a unified physics, in which the same laws applied to the celestial and terrestrial realms.[10] There were scientists who were qualified in both physics and astronomy who laid the firm foundation for the current science of astrophysics. In modern times, students continue to be drawn to astrophysics due to its popularization by the Royal Astronomical Society and notable educators such as prominent professors Lawrence Krauss, Subrahmanyan Chandrasekhar, Stephen Hawking, Hubert Reeves, Carl Sagan and Neil deGrasse Tyson. The efforts of the early, late, and present scientists continue to attract young people to study the history and science of astrophysics.[29][30][31]

See also


  1. ^ Keeler, James E. (November 1897), "The Importance of Astrophysical Research and the Relation of Astrophysics to the Other Physical Sciences", The Astrophysical Journal, 6 (4): 271–288, Bibcode:1897ApJ.....6..271K, doi:10.1086/140401, [Astrophysics] is closely allied on the one hand to astronomy, of which it may properly be classed as a branch, and on the other hand to chemistry and physics.… It seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space–what they are, rather than where they are.… That which is perhaps most characteristic of astrophysics is the special prominence which it gives to the study of radiation.
  2. ^ "astrophysics". Merriam-Webster, Incorporated. Archived from the original on 10 June 2011. Retrieved 2011-05-22.
  3. ^ a b "Focus Areas  - NASA Science".
  4. ^ "astronomy". Encyclopædia Britannica.
  5. ^ Lloyd, G.E.R. (1968). Aristotle: The Growth and Structure of His Thought. Cambridge: Cambridge University Press. pp. 134–5. ISBN 978-0-521-09456-6.
  6. ^ Cornford, Francis MacDonald (c. 1957) [1937]. Plato's Cosmology: The Timaeus of Plato translated, with a running commentary. Indianapolis: Bobbs Merrill Co. p. 118.
  7. ^ Galilei, Galileo (1989-04-15), Van Helden, Albert, ed., Sidereus Nuncius or The Sidereal Messenger, Chicago: University of Chicago Press (published 1989), pp. 21, 47, ISBN 978-0-226-27903-9
  8. ^ Edward Slowik (2013) [2005]. "Descartes' Physics". Stanford Encyclopedia of Philosophy. Retrieved 2015-07-18.
  9. ^ Westfall, Richard S. (1983-04-29), Never at Rest: A Biography of Isaac Newton, Cambridge: Cambridge University Press (published 1980), pp. 731–732, ISBN 978-0-521-27435-7
  10. ^ a b Burtt, Edwin Arthur (2003) [First published 1924], The Metaphysical Foundations of Modern Science (second revised ed.), Mineola, NY: Dover Publications, pp. 30, 41, 241–2, ISBN 9780486425511
  11. ^ Ladislav Kvasz (2013). "Galileo, Descartes, and Newton – Founders of the Language of Physics" (PDF). Institute of Philosophy, Academy of Sciences of the Czech Republic. Retrieved 2015-07-18.
  12. ^ Case, Stephen (2015), "'Land-marks of the universe': John Herschel against the background of positional astronomy", Annals of Science, 72 (4): 417–434, Bibcode:2015AnSci..72..417C, doi:10.1080/00033790.2015.1034588, The great majority of astronomers working in the early nineteenth century were not interested in stars as physical objects. Far from being bodies with physical properties to be investigated, the stars were seen as markers measured in order to construct an accurate, detailed and precise background against which solar, lunar and planetary motions could be charted, primarily for terrestrial applications.
  13. ^ Donnelly, Kevin (September 2014), "On the boredom of science: positional astronomy in the nineteenth century", The British Journal for the History of Science, 47 (3): 479–503, doi:10.1017/S0007087413000915
  14. ^ Hearnshaw, J.B. (1986). The analysis of starlight. Cambridge: Cambridge University Press. pp. 23–29. ISBN 978-0-521-39916-6.
  15. ^ Kirchhoff, Gustav (1860), "Ueber die Fraunhofer'schen Linien", Annalen der Physik, 185 (1): 148–150, Bibcode:1860AnP...185..148K, doi:10.1002/andp.18601850115
  16. ^ Kirchhoff, Gustav (1860), "Ueber das Verhältniss zwischen dem Emissionsvermögen und dem Absorptionsvermögen der Körper für Wärme und Licht", Annalen der Physik, 185 (2): 275–301, Bibcode:1860AnP...185..275K, doi:10.1002/andp.18601850205
  17. ^ Cortie, A. L. (1921), "Sir Norman Lockyer, 1836 – 1920", The Astrophysical Journal, 53: 233–248, Bibcode:1921ApJ....53..233C, doi:10.1086/142602
  18. ^ Jensen, William B. (2004), "Why Helium Ends in "-ium"" (PDF), Journal of Chemical Education, 81 (7): 944–945, Bibcode:2004JChEd..81..944J, doi:10.1021/ed081p944
  19. ^ Hetherington, Norriss S.; McCray, W. Patrick, Weart, Spencer R., ed., Spectroscopy and the Birth of Astrophysics, American Institute of Physics, Center for the History of Physics, archived from the original on September 7, 2015, retrieved July 19, 2015
  20. ^ a b Hale, George Ellery (1895), "The Astrophysical Journal", The Astrophysical Journal, 1 (1): 80–84, Bibcode:1895ApJ.....1...80H, doi:10.1086/140011
  21. ^ The Astrophysical Journal. 1 (1).
  22. ^ Eddington, A. S. (October 1920), "The Internal Constitution of the Stars", The Scientific Monthly, 11 (4): 297–303, JSTOR 6491
  23. ^ Eddington, A. S. (1916). "On the radiative equilibrium of the stars". Monthly Notices of the Royal Astronomical Society. 77: 16–35. Bibcode:1916MNRAS..77...16E. doi:10.1093/mnras/77.1.16.
  24. ^ Payne, C. H. (1925), Stellar Atmospheres; A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars (PhD Thesis), Cambridge, Massachusetts: Radcliffe College, Bibcode:1925PhDT.........1P
  25. ^ Haramundanis, Katherine (2007), "Payne-Gaposchkin [Payne], Cecilia Helena", in Hockey, Thomas; Trimble, Virginia; Williams, Thomas R., Biographical Encyclopedia of Astronomers, New York: Springer, pp. 876–878, ISBN 978-0-387-30400-7, retrieved July 19, 2015
  26. ^ Biermann, Peter L.; Falcke, Heino (1998), "Frontiers of Astrophysics: Workshop Summary", in Panvini, Robert S.; Weiler, Thomas J., Fundamental particles and interactions: Frontiers in contemporary physics an international lecture and workshop series. AIP Conference Proceedings, 423, American Institute of Physics, pp. 236–248, arXiv:astro-ph/9711066, Bibcode:1998AIPC..423..236B, doi:10.1063/1.55085, ISBN 1-56396-725-1
  27. ^ Roth, H. (1932), "A Slowly Contracting or Expanding Fluid Sphere and its Stability", Physical Review, 39 (3): 525–529, Bibcode:1932PhRv...39..525R, doi:10.1103/PhysRev.39.525
  28. ^ Eddington, A. S. (1988) [1926], Internal Constitution of the Stars, New York: Cambridge University Press, ISBN 978-0-521-33708-3
  29. ^ D. Mark Manley (2012). "Famous Astronomers and Astrophysicists". Kent State University. Retrieved 2015-07-17.
  30. ^ The team (2015). "Hubert Reeves – Astronomy, Astrophysics and Space Science". GCS Research Society. Retrieved 2015-07-17.
  31. ^ "Neil deGrasse Tyson". Hayden Planetarium. 2015. Retrieved 2015-07-17.

Further reading

External links

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