Nebulae and star clusters

At a glance

Values come from NASA Hubble/Webb pages (M42 size, NGC 1999, Pillars, M57, M1), Menten et al. 2007 (M42 parallax), Lodieu et al. 2019 (Pleiades), Gaia Collaboration 2018 (47 Tuc age) and the Harris catalogue, 2010 edition (47 Tuc distance; the half-light radius in parsecs is our conversion of its angular value). The Pillars’ height is from NASA’s Hubble Messier 16 page (which gives 7,000 light-years for the distance; the Webb release gives 6,500). NASA’s popular pages round M42 to 1,500 light-years. Checked 29 October 2026.

Sources: [1] [2] [3] [4] [5] [6] [7] [8] [9]

What nebulae are and how they shine

A nebula is a cloud of gas and dust, mostly hydrogen and helium with a small admixture of fine dust grains. What we see depends on how it interacts with starlight. This page covers the objects themselves; the diffuse medium between them has its own dossier.

Emission nebulae, above all H II regions, glow by themselves. Ultraviolet light from hot, massive young stars strips electrons from hydrogen atoms; when electrons recombine they cascade down the energy levels and emit light at fixed wavelengths. Hydrogen-alpha (Hα) gives these regions their red colour, while the “forbidden” line of doubly ionised oxygen, [O III], which forms only in extremely thin gas, adds blue-green. In the Orion Nebula the four Trapezium stars, each about 15–30 times the Sun’s mass, supply most of the ionising light.

Reflection nebulae shine only because dust scatters light from a nearby star that is not hot enough to ionise the gas. Grains scatter blue light more efficiently than red, so they look bluish; seen from the other side, the same physics causes extinction, dimming and reddening stars behind a dusty cloud. When the dust is dense enough to block background light completely we see a dark nebula, the “hole in the sky” that William Herschel described in the late 1700s. Small, cold, dense dark clouds are called Bok globules; some contain warm cores that betray star formation inside.

Sources: [1]

Where stars are born: molecular clouds, embedded clusters and associations

Dark nebulae are the visible tips of molecular clouds, the cold, dense phase in which hydrogen is molecular and stars form. A region collapses when its self-gravity overcomes its thermal pressure. The classical form of this condition is the Jeans criterion: a clump is unstable if its mass exceeds the Jeans mass, which grows with temperature (as T^3/2) and falls with density (as ρ^−1/2). Real clouds are messier: as McKee and Ostriker (2007) review, turbulence both creates collapsing over-densities and resists gravity, and magnetic fields matter too.

Stars often form in groups. Infrared surveys reveal embedded clusters: young stars still wrapped in their parent gas. Lada and Lada’s review found that the birth rate of embedded clusters exceeds that of visible open clusters by an order of magnitude or more, and that fewer than 4–7% survive as bound clusters to the age of the Pleiades. The unbound groups of young stars left behind are called stellar associations (OB associations when they contain massive stars).

The Pillars of Creation in the Eagle Nebula show the process at work: dense knots inside the columns collapse and ignite, and Webb sees stars only a few hundred thousand years old driving jets into the surrounding gas.

Sources: [4] [13]

Interactive lab: when does a cloud collapse?

The lab opens on a dense cloud core (10 K, 10⁵ cm⁻³): notice that the Jeans mass is under 2 solar masses and the collapse takes about a hundred thousand years, so such cores can give birth to individual stars. Then try the “Giant molecular cloud” and “Warm neutral gas” presets: as the gas gets warmer and thinner, the mass needed for collapse rises to about 100 solar masses, and in warm neutral gas to tens of millions.

Where stars die: planetary nebulae and supernova remnants

A planetary nebula has nothing to do with planets; the name comes from early telescopic impressions. It forms when a low- or intermediate-mass star (roughly 0.8 to 8 solar masses according to the review by Kwitter and Henry) sheds its outer layers at the end of its life. The exposed hot core ionises the gas and becomes a white dwarf, about a solar mass in a body the size of Earth. The Sun will follow this path.

In the Ring Nebula (M57), about 1 light-year across, Hubble found a doughnut of material around a football-shaped inner region seen nearly end-on. Its gas was expelled about 4,000 years ago, is expanding at more than 69,000 km/h, and will keep expanding for only about another 10,000 years: planetary nebulae are fleeting.

More massive stars end as supernovae, as can white dwarfs in binary systems that pull in matter from a companion. The fast-moving debris forms a supernova remnant glowing in X-rays and visible light. The Crab Nebula (M1) is the remnant of the “guest star” Chinese astronomers recorded in 1054, which was visible in daylight for weeks. Its central pulsar, a spinning neutron star, flashes about 30 times a second, and electrons spiralling in its magnetic field at nearly the speed of light make the nebula’s blue glow.

Sources: [1] [5] [6]

Open clusters: the Pleiades, the Hyades and the Gaia census

Open clusters are groups of tens to thousands of stars born together and loosely bound by gravity; Galactic tides slowly strip them. The Pleiades (M45) is the most famous: Lodieu and colleagues placed it at 135.15 ± 0.43 pc using Gaia DR2 data, counted 1,248 sources within its 11.6 pc tidal radius and derived an age of about 132 million years from its white-dwarf member.

The Hyades in Taurus is much closer still. The same team, again using Gaia DR2, found 710 candidate members within 30 pc of its centre, placed the centre at 47.03 ± 0.20 pc and estimated an age of about 640 million years from its white dwarfs.

Using Gaia DR3, Hunt and Reffert (2023) recovered 7,167 clusters across the whole sky, of which 2,387 were new candidates and 4,105 formed a highly reliable subset. They cautioned that up to a few thousand may be unbound moving groups rather than true clusters, so the count depends on the definition.

Sources: [7] [10] [11]

Globular clusters: ancient, dense and not as simple as once thought

Globular clusters are dense, nearly spherical swarms of hundreds of thousands of stars in the Milky Way’s halo and bulge, among its oldest objects: in Gaia’s observational HR diagrams, 47 Tucanae is fitted with an age of 12.75 billion years. The Harris catalogue (2010 edition) lists 157 Milky Way globular clusters; Holger Baumgardt’s globular cluster database (version 4, March 2023) includes 168. The total is thus roughly 150–170, depending on how uncertain candidates are treated.

They were long treated as “simple stellar populations” of one age and composition. Spectroscopy and precise photometry instead show star-to-star variations in helium, carbon, nitrogen, oxygen, sodium and aluminium, carrying the signature of hydrogen burning at high temperature. According to Bastian and Lardo’s review, such multiple populations are seen in nearly all old clusters but have not been found in any massive cluster younger than about 2 billion years.

Sources: [8] [9] [12]

Clusters as clocks and rulers

A cluster’s stars share a distance, an age and roughly a composition, so its Hertzsprung–Russell (HR) diagram is a controlled experiment. Massive stars exhaust their core hydrogen first, so the main sequence peels away from the top; the turn-off point moves to fainter, redder stars with time and acts as a clock. Fitting theoretical isochrones to Gaia’s HR diagrams gives, for example, about 3.5 billion years for the old open cluster M67 and 12.75 billion years for 47 Tucanae. White-dwarf ages for the Pleiades and Hyades provide independent checks.

Clusters also anchor the distance ladder. With a nearby cluster’s distance fixed by parallax, comparing its main sequence with those of farther clusters (main-sequence fitting) yields their distances. The Hyades and Pleiades long played this role; Gaia now measures parallaxes for thousands of clusters directly.

Sources: [7] [8] [10]

Hubble and Webb views

Hubble images of the Orion Nebula, about 15 light-years across, reveal thousands of developing stars and newborn planetary systems in the making. In the Ring Nebula, Hubble revealed the three-dimensional shape behind the familiar ring.

The James Webb Space Telescope (JWST) sees through dust in the infrared. Its first-release image of 12 July 2022 showed the “Cosmic Cliffs” of NGC 3324 at the north-west corner of the Carina Nebula, roughly 7,600 light-years away: the edge of a cavity carved by radiation and winds from massive young stars, with peaks about 7 light-years high and protostellar jets. On 19 October 2022 Webb’s NIRCam returned a new view of the Pillars of Creation, with newly formed stars and wavy, lava-like edges where jets from forming stars strike the gas.

Sources: [3] [4] [5] [14]

Catalogues, data archives and observing from Türkiye

The Messier catalogue of 110 objects was compiled by the French comet hunter Charles Messier (1730–1817) to help comet hunters avoid false alarms; Hubble has imaged 89 of them (NASA Hubble Messier catalogue). The far larger New General Catalogue (NGC) and its supplements, the Index Catalogues (IC), supply most other designations. Research relies on specialised catalogues: Harris and Baumgardt for globular clusters, and Gaia-based ones such as Hunt and Reffert (2023) for open clusters.

Hubble and JWST observations are in MAST, the Mikulski Archive for Space Telescopes. Gaia data are in the ESA Gaia Archive; as of October 2026 ESA has scheduled Gaia DR4 for 2 December 2026, with the archive offline from 25 November (ESA Gaia news 2026). The CDS in Strasbourg hosts SIMBAD (identifications and bibliography for objects) and VizieR, where most published catalogues, including those above, can be queried.

For observers in Türkiye, the winter sky is richest: NASA lists January as the best month for the Orion Nebula and the Crab, and December for the Pleiades, which are easy to the naked eye. Summer brings the globular cluster M13 in Hercules (best in July, visible in binoculars) and the Eagle Nebula (August). 47 Tucanae lies in the far southern sky and cannot be seen from Türkiye. Narrowband Hα or [O III] filters help against light pollution.

Sources: [6] [9] [11]

Known vs open questions

Established

• Emission nebulae shine by recombination in gas ionised by hot stars; reflection nebulae by dust scattering; dark nebulae by dust absorption.
• Planetary nebulae come from low- and intermediate-mass stars and last only thousands to tens of thousands of years; the Crab is the remnant of SN 1054.
• Embedded clusters form at least ten times as often as visible open clusters, and only a few per cent of them survive as bound clusters.
• Globular clusters are among the Galaxy’s oldest objects (47 Tucanae: 12.75 billion years); Gaia gives precise distances to thousands of open clusters.

Open

• What produces multiple populations in globular clusters, and why are they absent in massive clusters younger than about 2 billion years?
• What sets the star formation rate in molecular clouds and the stellar initial mass function?
• Which of the thousands of Gaia-detected groups are genuinely bound clusters rather than dispersing moving groups?
• How much do binary companions shape planetary nebulae, as suggested for the Ring Nebula from Webb images?

Sources: [5] [6] [11] [12] [13]