Эрдэмтэд агаар мандлын найрлагаас шалтгаалан ногоон болон ягаан туяатай харагдах цагаан одой оддыг тодорхойлж, тэдгээрийн гэрлийн спектрийг загварчиллаа.
Уорвикийн их сургуулийн судлаачид WD J1330+6435 хэмээх цагаан одой од нь хүний нүдээр харвал цайвар ногоон өнгөтэй харагдах болохыг тогтоожээ. Нар шиг оддын түлш дууссаны дараа үлддэг нягт цөм болох энэ од нь 4000 орчим хэмийн халуунтай бөгөөд агаар мандалд нь агуулагдах натри, кальци зэрэг элементийн нөлөөгөөр гэрлийн спектр нь өөрчлөгддөг байна. Судлаачид спектрийн ажиглалт болон агаар мандлын загварчлалыг хүний өнгө мэдрэх чадварын стандарт загвартай хослуулан ийм дүгнэлтэд хүрчээ.
Ихэнх оддын өнгө нь температурын үзүүлэлтээс хамаардаг бол цагаан одой оддын хувьд нягт ихтэй агаар мандал нь гэрлийг өргөн хүрээнд шингээснээр өнгийг нь эрс өөрчилдөг. Тухайлбал, WD J1330+6435 одонд агуулагдах натри нь улбар шар өнгийн гэрлийг, кальци нь ягаан туяаг тус тус шингээснээр ногоон өнгө давамгайлах нөхцөлийг бүрдүүлдэг байна. Судалгааны баг мөн WD J2356–2054 хэмээх од нь 4000 хэмийн халуун хэдий ч тод цэнхэр өнгөтэй харагдах боломжтойг тогтоожээ.
Түүнчлэн судлаачид дэлхийгээс 48 гэрлийн жилийн зайд орших WD J1011+2845 одыг өнгөний хувьд хамгийн улаан од байж болзошгүй гэж үзэв. Энэ одны агаар мандалд нүүрстөрөгчийн молекулууд үүссэнээр 400-600 нанометрийн хоорондох гэрлийг хүчтэй шингээж, улмаар гүн ягаан-улбар шар (salmon-pink) өнгөтэй болгодог байна.
Судлаачид Нарнаас 65 гэрлийн жилийн зайд орших цагаан одой оддыг судалж үзэхэд ойролцоогоор 3.4 хувь нь буюу 30 од тутмын нэг нь ер бусын өнгөтэй болохыг тогтоожээ. Гэсэн хэдий ч үлдсэн цагаан одой оддын ихэнх нь цагаан өнгөтэй байдаг нь тэдгээрийн гадаргуугийн температурын ерөнхий шинж чанартай нийцэж байна. Энэхүү судалгааны үр дүн нь оддын агаар мандлын найрлага болон тэдгээрийн харагдах байдалд үзүүлэх нөлөөг илүү тодорхой болгож өгчээ.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
About 4,000 degrees Celsius (7,200 °F) is cool for a star. An ordinary star at that temperature should look orange. But according to astronomers at the University of Warwick, a stellar remnant called WD J1330+6435 would appear a pale, ghostly green if you could view it from close enough.
It is a white dwarf, the dense core left behind after a star like the Sun exhausts its nuclear fuel and sheds its outer layers. Its green appearance has not been observed directly by eye. Instead, researchers reconstructed its perceived color from its spectrum, the pattern of light it emits across different wavelengths.
The team combined spectral observations and atmospheric models with a standard model of human color vision to calculate how white dwarfs would look up close. For WD J1330+6435, the result was chartreuse, a yellowish green.
According to the university’s statement, the researchers know of no previous credible report of a star with an intrinsically green appearance. Their analysis also identified two paler green white dwarfs and what may be the reddest-looking star known. The explanation lies in how these stars’ dense atmospheres absorb light.
Why Most Stars Look Orange, White or Blue
For most stars, color is largely a readout of temperature. Cooler stars look orange, stars like the Sun look white, and the hottest look pale blue. Their overall colors resemble those of ideal thermal emitters, known as blackbodies. Even when a star’s spectrum peaks at green wavelengths, the accompanying red and blue light usually combines to produce a different perceived color.
White dwarf atmospheres can alter that balance dramatically. In the coolest examples, atmospheric density can reach about 1 gram per cubic centimeter, roughly the density of water and millions of times denser than the Sun’s visible surface. Under these conditions, atoms and molecules absorb light across much broader wavelength ranges than they would in thinner gas.
Lead author Mark Hollands and his colleagues examined dozens of white dwarfs spanning different temperatures and atmospheric compositions. Carbon-rich examples often appeared pink or purple, while some stars polluted by rocky debris looked yellow. Certain cool white dwarfs with mixed hydrogen and helium atmospheres took on cyan hues.
How Rocky Debris Gives a White Dwarf Its Green Color
WD J1330+6435 belongs to the polluted group. It has accumulated rocky material from its planetary system, adding calcium, lithium and sodium to its atmosphere. The presence of lithium had already drawn researchers’ attention in 2021. In the new analysis, the team fitted an atmospheric temperature of 4,240 K, equivalent to about 4,000 °C.
Its unusual color comes from the light those elements remove. Broad absorption by sodium suppresses orange wavelengths, while calcium absorbs toward the violet end of the spectrum. The resulting spectrum peaks near 500 nanometers, producing the calculated yellowish-green appearance. “Imagine standing on a planet and looking up at a green star,” Hollands said in the university’s statement.
The researchers acknowledge some uncertainty in the exact shade because the observed spectrum is noisy. However, an earlier atmospheric model using a simpler treatment of sodium absorption also produced a green hue. Two other white dwarfs in the study, WD J0851+1543 and WD J1351+2645, appeared green as well, although much paler.
An even more extreme example is WD J2356–2054. Despite a temperature of only about 4,000 K (3,700 °C), it would appear vivid blue. Exceptionally strong sodium absorption suppresses much of the middle of its visible spectrum, leaving its perceived color dominated by shorter wavelengths. Its calculated blue lies beyond the range that a blackbody could produce at any temperature.
A Nearby White Dwarf May Be the Reddest-Looking Star Known
At the other extreme is WD J1011+2845, a white dwarf about 48 light-years from Earth. Carbon brought up from its interior forms molecules in its dense atmosphere. Their absorption bands, distorted by the dense gas and likely a strong magnetic field, greatly suppress light between 400 and 600 nanometers, leaving most of its visible output at longer wavelengths.
The resulting color is a distinctive deep salmon-pink. The researchers describe it as likely the reddest star known in terms of perceived color. That qualification matters: they are describing how its visible light would appear to human eyes, rather than simply identifying the coolest star or the one emitting the most infrared radiation.
To investigate how common unusual colors might be, the team also examined the known white dwarfs within 20 parsecs, about 65 light-years, of the Sun. Five stood out sufficiently from the usual blackbody color range: two yellow and three pink. The authors estimate that about 3.4%, or roughly 1 in 30, have unusually colored light by their chosen threshold.
For the remaining stars, the median color temperature was approximately 6,800 K, corresponding to a nearly white appearance. Color temperature describes the hue of the light by comparison with a blackbody; it is distinct from the star’s actual surface temperature.
Hotter, bluer white dwarfs are brighter and easier to detect at greater distances, which helps explain their familiar blue depiction. The nearby sample gives a different picture: the typical white dwarf really does look white.
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