Editing Radiation pressure (section)

== Cosmic effects of radiation pressure ==

Radiation pressure has had a major effect on the development of the cosmos, from the birth of the universe to ongoing formation of stars and shaping of clouds of dust and gasses on a wide range of scales.<ref>{{Citation|last=Karel Velan | first=A. | chapter=The Birth of the First Generation of Stars | date=1992 | pages=267–278 | publisher=Springer US | isbn=9781468460322 | doi=10.1007/978-1-4684-6030-8_22 | title=The Multi-Universe Cosmos}}</ref>

=== Early universe ===

The [[photon epoch]] is a phase when the energy of the universe was dominated by photons, between 10 seconds and 380,000 years after the [[Big Bang]].<ref>{{Cite book|title=The early universe|date=1988|publisher=D. Reidel| others=Unruh, W. G., Semenoff, G. W., North Atlantic Treaty Organization. Scientific Affairs Division.| isbn=9027726191| location=Dordrecht| oclc=16684785}}</ref>

=== Galaxy formation and evolution ===
[[File:Pillars of creation 2014 HST WFC3-UVIS full-res.jpg|thumb|The ''Pillars of Creation'' clouds within the [[Eagle Nebula]] shaped by radiation pressure and stellar winds.]]The process of [[galaxy formation and evolution]] began early in the history of the cosmos. Observations of the early universe strongly suggest that objects grew from bottom-up (i.e., smaller objects merging to form larger ones). As stars are thereby formed and become sources of electromagnetic radiation, radiation pressure from the stars becomes a factor in the dynamics of remaining [[Circumstellar disc|circumstellar]] material.<ref>{{cite book| title=Galaxy formation| last=Longair, Malcolm S., 1941–| date=2008| publisher=Springer| isbn=9783540734772| oclc=212409895}}</ref>

=== Clouds of dust and gases ===
The [[gravitational compression]] of clouds of dust and gases is strongly influenced by radiation pressure, especially when the condensations lead to star births.  The larger young stars forming within the compressed clouds emit intense levels of radiation that shift the clouds, causing either dispersion or condensations in nearby regions, which influences birth rates in those nearby regions.

=== Clusters of stars ===
<!--[[File:M92 arp 750pix.jpg|thumb|250px|Star cluster [[Messier 92]].]]-->
Stars predominantly form in regions of large clouds of dust and gases, giving rise to [[star cluster]]s.  Radiation pressure from the member stars eventually disperses the clouds, which can have a profound effect on the evolution of the cluster.

Many [[open cluster]]s are inherently unstable, with a small enough mass that the [[escape velocity]] of the system is lower than the average [[velocity]] of the constituent stars. These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.[[File:David A. Aguilar's Red Dwarf Stars.jpg|thumb|A protoplanetary disk with a cleared central region (artist's conception).]]

=== Star formation ===

[[Star formation]] is the process by which dense regions within [[molecular cloud]]s in [[interstellar space]] collapse to form [[star]]s. As a branch of [[astronomy]], star formation includes the study of the [[interstellar medium]] and [[giant molecular cloud]]s (GMC) as precursors to the star formation process, and the study of [[protostar]]s and [[young stellar object]]s as its immediate products.  Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of [[binary star]]s and the [[initial mass function]].

=== Stellar planetary systems ===
[[Planetary system]]s are generally believed to form as part of the same process that results in [[star formation]]. A [[protoplanetary disk]] forms by gravitational collapse of a [[molecular cloud]], called a [[solar nebula]],  and then evolves into a planetary system by collisions and gravitational capture. Radiation pressure can clear a region in the immediate vicinity of the star. As the formation process continues, radiation pressure continues to play a role in affecting the distribution of matter.  In particular, dust and grains can spiral into the star or escape the stellar system under the action of radiation pressure.[[File:Comet Hale-Bopp 1995O1.jpg|thumb|[[Comet Hale–Bopp]] (C/1995 O1). Radiation pressure and solar wind effects on the dust and gas tails are clearly seen.]]

=== Stellar interiors ===

In [[star|stellar]] interiors the temperatures are very high. Stellar models predict a temperature of 15&nbsp;MK in the center of the [[Sun]], and at the cores of [[supergiant]] stars the temperature may exceed 1&nbsp;GK. As the radiation pressure scales as the fourth power of the temperature, it becomes important at these high temperatures. In the Sun, radiation pressure is still quite small when compared to the gas pressure. In the heaviest non-degenerate stars, radiation pressure is the dominant pressure component.<ref>Dale A. Ostlie and Bradley W. Carroll, ''An Introduction to Modern Astrophysics'' (2nd edition), page 341, Pearson, San Francisco, 2007</ref>

=== Comets ===
Solar radiation pressure strongly affects [[comet tail]]s.  Solar heating causes gases to be released from the [[comet nucleus]], which also carry away dust grains.  Radiation pressure and [[solar wind]] then drive the dust and gases away from the Sun's direction.  The gases form a generally straight tail, while slower moving dust particles create a broader, curving tail.