Astronomers say they’ve discovered the first green star

Courtesy Mark A. Garlick/University of Warwick

Astronomers analyzing white dwarf stars have for the first time found a star that would appear green to human eyes, according to a new study.

While for most stars, color is temperature-dependent and limited to a few hues, white dwarfs, which are the dense remnants of sun-like stars that have died, can glow in an incredible range of colors, said lead study author Mark Hollands, an astrophysicist at the University of Warwick, England, on Thursday.

“The colors that normal stars can have are pretty much only orange, white or blue,” Hollands told CNN.

“What we found is, for most white dwarfs, that’s the same as well. But there are some special cases where the physics in the outer layers of these stars are sufficiently strange that it causes actually quite big deviations in terms of their spectrum,” he said.

Courtesy Mark Hollands/University of Warwick

These deviations are due to the interaction between the helium in their atmosphere and traces of other elements, which vary between white dwarfs, depending on what material has interacted with the star over the course of its lifetime.

For instance, iron, magnesium and sodium, elements commonly found in rocks, can be found on the surface — potential evidence of a nearby former planetary system, or from “something like an asteroid that ventures too close to the white dwarf,” Hollands explained.

“It gets shredded up into dust, and then those rocky bits of dust and debris fall onto the white dwarf,” he said.

“And then that gets mixed into the outer layers, and then you can actually use that to measure what the rocky stuff was made off,” said Hollands.

“So we can actually use the white dwarf as a sort of diagnostic tool to actually measure the chemistry of planetary material,” he added.

“The white dwarf is taking apart the planets, and we’re able to see what was actually inside that planet — the proportion of the different elements — and that gives us clues in how these planets were formed,” said Adam Burgasser, a professor of astronomy and astrophysics at the University of California San Diego. “It is a crime scene, and we’re kind of piecing through the evidence that we have left available to us, and that’s allowing us to kind of piece the story of what the planet was and how it was made way, way, way back in in history.”

For this study, Hollands looked at the 151 known white dwarf stars that are within 20 parsecs, or around 65 light years, of Earth.

As for the green star, the main elements present are calcium, sodium and lithium, Hollands said.

“It kind of stimulated interest in me in looking more at that object and why it has so much material being accreted onto it,” said Burgasser, who was not involved in the study. “It can it can excite the public as well as the scientists in learning more about some of the more, I would say, extreme objects in this sample.”

Another notable star was “almost like a salmon pink,” due to the presence of a high density of carbon molecules, Hollands added.

While no white dwarf stars can be seen directly from Earth, Hollands was able to calculate their color using star spectrum data, which a statement from the researchers defines as “the ‘fingerprint’ produced when starlight is split apart by wavelength, much like a prism splits sunlight into a rainbow.”

Hollands explained that the research has also brought these space objects much closer to home.

“It gives you a nice human connection to these very distant things that are so far away that sometimes even as a researcher you feel quite disconnected from these objects that you study,” he said. “But then you can sort of add a human element into it and it feels a lot more tangible.”

The observations about green star are based on existing data and may not shed new understandings about white dwarfs, but the study does offer an intriguing approach and view of these celestial bodies, said Agnès Kim, an astrophysicist and professor of physics at Penn State Scranton, who was not involved in the work.

“Doing a study that goes from spectrum to what the human eye would see is a cool intersection of physics and biology to me,” Kim wrote in an email. “I love that some would look green to us if they were close enough for us to see them, and even more fun that some would look pink.”

Burgasser added that the study’s categorization scheme using temperatures and atmospheric properties can be useful for the field moving forward.

“Some things are kind of purplish pink, and some other things were greenish and blue — and that kind of classification, which by the way we do all the time in astronomy, is classify things, allows us to focus on: why are these types of objects these colors, and are they related to each other based on their composition?” Burgasser said. “We have that information in other forms, but I think this is maybe a simplified way of organizing that data.”

Next, Hollands is planning to keep analyzing more white dwarf star spectra, with advances in scientific techniques meaning that researchers will be able to look at a new tranche of stars that are even farther away from Earth.

“We know of white dwarfs that we don’t have spectra for,” he said, “but that’s all about to change because there’s quite a few spectroscopic surveys” that are going to start delivering data.

“We’ll be getting a tenfold increase in the number of white dwarfs that have spectra,” Hollands said. “And so maybe if we’re lucky, we’ll go from one or maybe two green white dwarfs to 10 or 20.”

The study was published in the journal RAS Techniques and Instruments, or RASTI.

图片由马克·A·加里克/华威大学提供

一项新的研究表明，天文学家通过分析白矮星，首次发现了一颗在人眼看来呈绿色的恒星。

该研究的主要作者、英国华威大学的天体物理学家马克·霍兰兹周四表示，虽然大多数恒星的颜色取决于温度，并且仅限于几种色调，但白矮星（类似太阳的恒星死亡后留下的致密残骸）可以发出令人难以置信的各种颜色。

霍兰兹告诉 CNN：“普通恒星的颜色基本上只有橙色、白色或蓝色。”

他说：“我们发现，对于大多数白矮星来说，情况也是如此。但也有一些特殊情况，这些恒星外层的物理性质非常奇特，以至于导致它们的光谱出现相当大的偏差。”

图片由马克·霍兰兹/华威大学提供

这些偏差是由于白矮星大气层中的氦与其他微量元素相互作用造成的，而这些元素的含量在白矮星之间有所不同，这取决于白矮星在其生命周期中与哪些物质发生了相互作用。

例如，铁、镁和钠这些岩石中常见的元素可以在地表发现——这可能是附近曾经存在行星系统的证据，或者来自“像小行星那样过于靠近白矮星的东西”，霍兰兹解释说。

“它会被撕成粉末，然后这些岩石状的尘埃和碎片会落到白矮星上，”他说。

“然后这些物质会混入外层，这样你就可以利用这些物质来测量岩石是由什么构成的，”霍兰兹说。

“所以我们实际上可以利用白矮星作为一种诊断工具来测量行星物质的化学成分，”他补充道。

“白矮星正在瓦解这些行星，我们得以窥见行星内部的真实构成——不同元素的比例——这为我们了解这些行星的形成提供了线索，”加州大学圣地亚哥分校天文学和天体物理学教授亚当·伯格瑟说道。“这就像一个犯罪现场，我们正在努力拼凑现有的证据，这让我们得以了解这些行星在遥远的过去是如何形成的。”

在这项研究中，霍兰兹研究了距离地球 20 秒差距（约 65 光年）以内的 151 颗已知的白矮星。

霍兰兹说，至于绿色的星星，其主要成分是钙、钠和锂。

“这激发了我进一步研究这个物体以及它为何会吸附如此多物质的兴趣，”未参与这项研究的伯格瑟说道。“它既能激发公众的兴趣，也能激发科学家们对样本中一些比较极端的物体的了解。”

霍兰兹补充说，另一颗引人注目的恒星“几乎像鲑鱼粉色”，这是由于碳分子密度很高造成的。

虽然从地球上无法直接看到白矮星，但霍兰兹能够利用恒星光谱数据计算出它们的颜色。研究人员在一份声明中将恒星光谱定义为“当星光按波长分解时产生的‘指纹’，就像棱镜将阳光分解成彩虹一样”。

霍兰兹解释说，这项研究也让这些太空物体离我们更近了。

他说：“它让你与这些遥远的事物建立起一种美好的人性联系，这些事物如此遥远，以至于有时即使作为研究人员，你也会感到与你研究的对象相当疏离。但当你把人性因素融入其中时，它就感觉更加真实可触了。”

宾夕法尼亚州立大学斯克兰顿分校的天体物理学家兼物理学教授阿格尼斯·金（Agnès Kim，她并未参与这项研究）表示，对绿星的观测是基于现有数据，可能不会带来关于白矮星的新认识，但这项研究确实提供了一种有趣的视角和方法来研究这些天体。

“对我来说，这项研究从光谱到人眼所见的光谱，是物理学和生物学之间一个很酷的交叉领域，”金在邮件中写道。“我喜欢有些光谱如果离我们足够近，看起来会是绿色的，而更有趣的是，有些光谱看起来会是粉红色的。”

Burgasser补充说，该研究利用温度和大气特性进行的分类方案对该领域的发展很有帮助。

“有些天体呈紫粉色，有些则呈绿色或蓝色——这种分类方法，也就是我们在天文学中经常使用的分类方法，使我们能够专注于：为什么这些天体呈现这些颜色，以及它们之间是否存在基于成分的关联？”伯格瑟说道。“我们以其他形式掌握了这些信息，但我认为这或许是一种更简化的数据整理方式。”

接下来，霍兰兹计划继续分析更多白矮星的光谱，随着科学技术的进步，研究人员将能够观察一批距离地球更远的恒星。

“我们知道有些白矮星我们没有光谱数据，”他说，“但这种情况即将改变，因为有很多光谱观测项目”即将开始提供数据。

霍兰兹说：“我们将获得光谱数据的白矮星数量增加十倍。所以，如果我们幸运的话，也许我们会从一两颗绿白矮星增加到十颗或二十颗。”

该研究发表在 RAS 技术与仪器杂志（RASTI）上。