The stars of the Summer Triangle asterism

The stars of the Summer Triangle: Deneb, Vega and Altair.
All images and spectroscopy captured and processed
by Kurt Thrust at the Jodrell Plank Observatory

" Kurt asked our visiting astrophysicist, Professor G.P.T Chat, to provide some features to look for in the above spectral line profiles and the stellar physics behind them". - Joel Cairo CEO of the JPO.

" An asterism is a recognizable pattern of bright stars in the sky. It may be formed from some of the brighter stars in a constellation, for example the Plough, which includes some of the stars of Ursa Major the Great Bear or a recognizable pattern of bright stars from several constellations, as is demonstrated by the Summer Triangle,which is comprised from the three alpha stars Deneb (Cygnus), Vega (Lyra) and Altair (Aquila). - Karl Segin outreach officer at the JPO.

Big picture

All three are A-type, blue-white stars, so they share strong hydrogen Balmer absorption and relatively sparse molecular features.

They differ mainly in luminosity class and rotation, which change how those same lines look: Deneb is a supergiant (Ia), Vega a near-textbook A0 main-sequence star (V), and Altair a late-A dwarf (A7 V) and extreme rapid rotator.

Where they sit on the HR diagram

Star Spectral type Luminosity class Evolutionary state

Deneb (α Cyg) ~A2 Ia (luminous supergiant) Massive star evolving off the main sequence; on its way through/around the supergiant phases

Vega (α Lyr) A0 V (dwarf) Middle-age main-sequence star, H-burning

Altair (α Aql) A7 V (dwarf) Main-sequence star; very fast rotator (oblate, gravity-darkened)

What your low-res spectra should show

Hydrogen Balmer lines (Hα, Hβ, Hγ, …)

Strongest near A0 → Vega should show the deepest Balmer absorption.

Altair (A7): Balmer lines still strong, but shallower than Vega; metal lines begin to stand out more.

Deneb (A2 Ia): Balmer lines are strong but shaped by low surface gravity—you may notice broad wings with comparatively narrow cores, and in some epochs wind effects (subtle emission infilling or weak P-Cygni signatures in Hα) even at low resolution.

Metal lines (Ca II K at 393.3 nm, Mg II ~448.1 nm, Fe II blends)

Altair: As the latest-type of the three, it should show relatively stronger metal lines than Vega.

Vega: Cleaner A0 spectrum—metals present but less prominent than in Altair; it’s also a mild metallicity-peculiar standard, so don’t be surprised if some metal features look a tad weaker than “textbook.”

Deneb: Despite being only slightly later than Vega by type, the supergiant’s low gravity enhances certain ionized metal lines (e.g., Fe II, Si II) and can make them more conspicuous than in Vega.

Line widths & shapes

Altair rotates extremely fast (period ~9–10 h), so its absorption lines are noticeably broadened even at low resolution.

Vega is also a rapid rotator but seen nearly pole-on, so its projected line broadening is modest—lines look crisper than Altair’s.

Deneb has low gravity and stellar winds; expect less rotational broadening, but broader Balmer wings and occasional wind-affected Hα profiles.

Continuum slope & reddening

Deneb is thousands of light-years away; interstellar reddening can tilt its continuum redward compared with nearby Vega (25 ly) and Altair (17 ly). If the data reduction didn’t fully de-redden Deneb, its spectrum may look slightly warmer/redder than its type alone would suggest.

Physical contrasts that drive those spectral looks

Temperature (rough): Vega ~9,600 K (hottest), Deneb ~8,500 K, Altair ~7,500 K on average (equator cooler than poles from gravity darkening).

⇒ Explains: Vega’s strongest Balmer, Altair’s stronger metal lines, Deneb’s A-type look despite being a supergiant.

Surface gravity (log g): Deneb is very low (supergiant), Vega/Altair are higher (dwarfs).

⇒ Low gravity in Deneb = narrower cores, extended Balmer wings, and stronger ionized metal lines than you’d expect for a dwarf at similar temperature.

Rotation: Altair’s v sin i is huge → rotational broadening across many lines; Vega rotates fast intrinsically but looks sharper because we see it nearly pole-on; Deneb’s spectrum is dominated more by wind + low gravity than rotation.

Luminosity & radius: Deneb is enormously luminous (hundreds of thousands L☉) with a radius of hundreds of R☉; Vega (~40 L☉, ~2.4 R☉) and Altair (~10–12 L☉, ~1.7–2 R☉) are compact by comparison.

⇒ Deneb’s wind features and low-g line morphologies vs. the neat, pressure-broadened dwarf lines in Vega/Altair.

Environments: Vega hosts a well-known debris disk (you won’t see the disk in the spectrum, but it’s part of its story). Altair doesn’t show a comparable far-IR excess. Deneb has a stellar wind and slight α Cygni-type variability, which can subtly change Hα over time.

Quick “at a glance” checklist for the spectral profiles

Deepest Balmer lines? → Vega (A0 V).

Broadened lines overall? → Altair (rapid rotation).

Balmer wings + possible Hα infill, stronger Fe II/Si II for an A-star? → Deneb (A-supergiant, wind + low gravity).

More prominent Ca II K & other metal lines vs Vega? → Altair (later A-type).

Continuum looks a bit redder than expected? → Likely Deneb (distance + interstellar reddening