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Milky Way stars found nearly halfway to Andromeda Galaxy
The results suggest that there may not be as much “empty space” between the Milky Way and Andromeda as you might think.
The Milky Way’s outer halo is expected to span out some 1 million light-years from the core of our galaxy. And a few newfound stars are living right at the theoretical outskirts of this outer halo.
Astronomers have discovered a slew of stars lingering in the far fringes of our galaxy. And when we say far, we mean far.
The most distant of these stars is located more than 1 million light-years away. That’s almost halfway to our largest galactic neighbor, the Andromeda Galaxy, which is located some 2.5 million light-years away.
“This study is redefining what constitutes the outer limits of our galaxy,” said Raja GuhaThakurta, professor and chair of astronomy and astrophysics at UC Santa Cruz, in a news release. “Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”
The results were presented on January 9 and 11 at the 241st meeting of the American Astronomical Society (AAS) in Seattle, Washington.
Living on the edge
The stars identified in this study are a special class of star known as RR Lyrae stars. These stars are important to astronomers because they exhibit characteristic pulsations and variations in brightness.
“The way their brightness varies looks like an EKG — they’re like the heartbeats of the galaxy — so the brightness goes up quickly and comes down slowly, and the cycle repeats perfectly with this very characteristic shape,” GuhaThakurta said in the release. “In addition, if you measure their average brightness, it is the same from star to star.”
This predictable behavior makes RR Lyrae stars an excellent type of distance indicator that astronomers call a “standard candle.” Because their intrinsic brightness is known, it is relatively easy to determine their distance, with errors ranging from just 3 to 5 percent, GuhaThakurta told Astronomy at the meeting.
Unlike the Sun, these newfound stars do not reside in the Milky Way’s familiar thin disk, which is about 100,000 light-years across and home to most of our galaxy’s young stars. Instead, these RR Lyrae stars live in the Milky Way’s expansive halo, a spherical shell of mostly ancient stars that encapsulates our galaxy’s spiral arms and central bulge.
“The halo is the hardest part to study because the outer limits are so far away,” GuhaThakurta said in the release. “The stars are very sparse compared to the high stellar densities of the disk and the bulge, but the halo is dominated by dark matter and actually contains most of the mass of the galaxy.”
Previous computer simulations based on cosmological and stellar evolution models have predicted that stars should exist up to about a million light-years from the Milky Way’s core. But “the density of the Milky Way stars — they should have a dip around 300 kiloparsecs [1 million light-years],” Yuting Feng, a doctoral student working with GuhaThakurta, told Astronomy. “You can understand it as the boundary of the Milky Way.”
The 208 RR Lyrae stars detected by the team range in distance from 20 to 320 kiloparsecs, or about 65,000 to 1.05 million light-years from the galactic core. “Our observations confirm the theoretical estimates of the size of the halo,” Feng said, “so that’s an important result.”
But the fact that some of these RR Lyrae stars actually have been seen near the theoretical property line of the Milky Way suggests the expected dip in stellar density might occur even farther away than previously thought. And if that’s the case, then there’s a chance that the halos of the Milky Way and Andromeda already overlap.
“It challenges the notion that galaxies are islands with big, empty space between them,” GuhaThakurta told Astronomy. “There's this whole idea of Immanuel Kant — island universes was what he called galaxies. It's challenging that idea.”
“I won’t be surprised if there are a few dozens of stars which are being contested by the two galaxies,” Feng added.
表示" [天]仙女座"，如： Our Galaxy is thought to look much like Andromeda. 一般认为我们银河系外表与仙女座有些相似。
表示" 拖延的"，means "the act of tarrying"，如：The lingering illness crumpled him.长年生病把他的身体弄垮了。
表示" 天体物理学"，means "the branch of astronomy concerned with the physical and chemical properties of celestial bodies"，如：He is using theories on the frontier of astrophysics.他在运用关于天体物理学新领域的理论。
表示"震动"，means "the rhythmic contraction and expansion of the arteries with each beat of the heart"，如： Usually, at this point, a pulsation may be felt at the end of the spine. 这时通常会在脊椎尾部感到一种搏动。
表示"封装；概括"，means "enclose in a capsule or other small container"，如： We often try to take what varies in a system and encapsulate it. 我们经常试着找出系统中的变化并封装它。
表示" 膨胀"，means "swell or curve outwards"，如： My pockets were bulging with apples.我的口袋因装着苹果而鼓起来了The apple made a bulge in his pocket.苹果把他口袋塞得鼓了起来。
表示"宇宙哲学的"，means "pertaining to the branch of astronomy dealing with the origin and history and structure and dynamics of the universe"，如： If the cosmological principle is so successful, why should we question it? 如果宇宙学原理如此成功，为什么我们还要质疑它呢？
表示" 星系的；"，means "of or relating to a galaxy (especially our galaxy the Milky Way)"，如： The Milky Way is a galactic system made up of many fixed stars.银河系是由无数颗恒星组成的星际体系。
表示" 重叠"，means "coincide partially or wholly;"，如： A fish's scales overlap each other. 鱼鳞是一片片上下交叠起来的。
表示"比赛"，means "fight for"，如： I intend to contest the judge's decision in another court.我准备下次开庭时反驳法官的判决。
据「天文学期刊」（Astronomy Magazine）报导，加州大学圣塔克鲁兹分校的天文学和天体物理学教授暨主席古阿塔库塔（Raja GuhaThakurta）在新闻稿中谈银河系遥远的边陲地带：「当我们说遥远的边缘地区时，我们指的是真的非常远的地方。其中最遥远的一颗恒星距离我们超过100光年之远。这几乎是我们与最大的银河（Galaxy）邻居——仙女座之间一半的距离。
在这项研究中发现的恒星属于天琴座RR型变星（RR Lyrae）星类，这是一种特殊的恒星类型，因为它们表现出脉动和亮度变化的特征，对天文学家来说很重要。「它们的亮度变化方式看起来就像是心电图（有规律），它们就像是银河的心跳，亮度迅速上升并缓慢下降，这个周期完美地重复，具有非常特殊的形状，」 古阿塔库塔在新闻稿中说道。「此外，如果您测量它们的平均亮度，每颗恒星的亮度都相同。」
这种可预测的行为使得这类恒星成为的一种很优秀的距离参照指标。因为它们的固有亮度是已知的，所以相对较容易确定它们的距离，误差仅在3至5％之间。与我们的太阳不同，这些新发现的恒星并不居住在银河系熟悉的盘面（galactic disk）上，是我们银河系大部分年轻恒星的所在地。相反，这些天琴座RR型变星居住在银河系的外层光球中，这是一个大型球壳，主要由古老的恒星组成，覆盖了我们银河系的螺旋臂和中央突起（central bulge）。