Editing Astronomical object (section)

== Within a galaxy ==
The constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the resulting fundamental components are the stars, which are typically assembled in clusters from the various condensing nebulae.<ref>{{cite conference | last=Elmegreen | first=Bruce G. | title=The nature and nurture of star clusters | book-title=Star clusters: basic galactic building blocks throughout time and space, Proceedings of the International Astronomical Union, IAU Symposium | volume=266 | pages=3–13 |date=January 2010 | doi=10.1017/S1743921309990809 | bibcode=2010IAUS..266....3E |arxiv = 0910.4638 }}</ref> The great variety of stellar forms are determined almost entirely by the mass, composition and evolutionary state of these stars. Stars may be found in multi-star systems that orbit about each other in a hierarchical organization. A planetary system and various minor objects such as asteroids, comets and debris, can form in a hierarchical process of accretion from the [[protoplanetary disk]]s that surround newly formed stars.

The various distinctive types of stars are shown by the [[Hertzsprung–Russell diagram]] (H–R diagram)—a plot of absolute stellar luminosity versus surface temperature. Each star follows an [[evolutionary track]] across this diagram. If this track takes the star through a region containing an [[intrinsic variable]] type, then its physical properties can cause it to become a [[variable star]]. An example of this is the [[instability strip]], a region of the H-R diagram that includes [[Delta Scuti variable|Delta Scuti]], [[RR Lyrae variable|RR Lyrae]] and [[Cepheid variable]]s.<ref name=hansen_et_al2004>{{cite book
 | first=Carl J. | last=Hansen |author2=Kawaler, Steven D.|author3= Trimble, Virginia
 | page=[https://archive.org/details/springer_10.1007-978-1-4419-9110-2/page/n98 86] | title=Stellar interiors: physical principles, structure, and evolution
 | url=https://archive.org/details/springer_10.1007-978-1-4419-9110-2 | series=Astronomy and astrophysics library
 | edition=2nd | publisher=Springer | date=2004
 | isbn=0-387-20089-4 }}</ref> The evolving star may eject some portion of its atmosphere to form a nebula, either steadily to form a [[planetary nebula]] or in a [[supernova]] explosion that leaves a [[supernova remnant|remnant]]. Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as a [[compact star|compact object]]; either a [[white dwarf]], [[neutron star]], or [[black hole]].