天空中那跳動的心 ∙ 上篇
Pumping hearts of the sky
你也許聽說過，我們都來自星塵，沒錯，這是真的。
As you may have heard before, we are made of star dusts, in fact that’s true.
宇宙中的物質總量總是守恆的，從聚集，變成物質，生存，死亡，分解再結合，所有的物質都在一個循環中。
他們中的一些聚集在一起組成你和我，其他的聚集在一起變成了星星。
Since the total amount of matter in the universe is always conserved, all the materials are in a cycle of gathering to become things, live, die, decompose and recombine. Some of them gathered to assemble you and me, the others gathered to form stars.
在他們變成我們在夜晚空中看到的星星之前，他們會首先相互吸引，成為分子雲。
經過數百萬年的積累，從星雲到原恆星，他們最終進入主序星的階段，仿佛一個孩子終於走進了人生的下一個階段，進入了成人世界。
Before turning into the stars that we see in the sky, the matter first attracts each other to become molecular clouds. After millions of years of accumulation, from nebulas to protostars, they finally enter stage of being a main sequence star, like a child walking into the adult world, ready for the next stage of their life.
Image of Eagle Nubula taken by Hubble from NASA
當恆星進化到了主序階段，它們的核開始燃燒氫，產生了向外的能量。由於產生能量的不連貫，恆星會膨脹和收縮，由此亮度也會變化。而這種現象的研究被稱為星震學，它能幫助科學家從恆星的表面變化來探索恆星的內部。
As stars evolve on to the main sequence, they start to have hydrogen burning in their cores, producing energy that is carried outwards. Due to the difference in the amounts of energy produced in a period of time, the star expands and contracts, causing the brightness to vary. The study of this phenomenon is called asteroseismology, which helps scientist to study inside of stars from their surface changes.
不同質量和年齡的恆星會有各自振動方式。Delta Scuti (δ Scuti) 星，大約質量為1.5-2.5倍太陽質量的主序星，是一群特殊的恆星。它們經常出現在銀河系裡的各種地方（比如：大麥哲倫星雲、球狀星團、疏散星團、銀河系中心、等等)。
所以通過觀察這些 δ Scuti) 星，科學家能夠找到強振動和亮度之間的關係，幫助他們計算出這些恆星離我們的距離，更加了解銀河系裡每個位置。
Stars of different masses and ages all oscillate differently. One class of stars in the main sequence with ~1.5 -2.5 solar mass are called the delta Scuti (δ Scuti) stars. By observing these δ Scutis, scientist would be able to find the relationship between their strongest oscillations and their luminosity, allowing them to calculate the distance to those stars, which appear frequently in the different parts on the milky way (e.g. the Large Magellanic Cloud, globular clusters, open clusters, the Galactic Center).
Image of the Large Magellanic Cloud taken by Hubble from NASA
要真正了解恆星的振動和它們的基本屬性，除了知道它們光度變化的周期，還需要更多細節。要做的就是根據振動發生的頻率來分類它們。類太陽的恆星就是這樣分析的，但那是因為它們的振動頻率分布平均。然而在克卜勒任務中發現的δ Scuti裡，它們的振動發生頻率幾乎隨機，毫無規律。這樣，星震學就無法算出需要研究的恆星的任何信息。
To really understand oscillating stars and their fundamental properties, more details need to be known than just the period-luminosity relations. Oscillations need to be categorized by their frequency of happening. This is often done to solar-like stars, which appear to have oscillations are evenly spread frequencies, however this is not the case with δ Scuti stars observed by the Kepler mission. The data showed the oscillations to be distributed almost randomly, meaning it’s impossible to asteroseismology to carried out on δ Scutis.
新任務，新出發
New Mission, new Way
為了解決這個問題，一組在雪梨的科學家開始了尋找規律振動δ Scuti星的奇妙之旅。雖然剛才說克卜勒發現的大多數恆星都只會隨機振動，但也發現過少數的有規則頻率的恆星。因此，要想觀察到規律振動的恆星，第一步就是增加模型中的樣本數量，而2018年由SpaceX獵鷹9號火箭發射的TESS任務在這裡就非常適合。
In order to solve this problem, a group of scientists based in Sydney set out to find regular oscillations of these δ Scuti stars. Although many found had only random oscillations, stars with regular spacings have also been found before, just with limited numbers. So, for regular patterns into be seen, the first step would be to increase the number of samples in the model and the TESS mission launched in 2018 with a SpaceX Falcon 9 rocket was the perfect fit in this case.
TESS 是一個凌日系外行星巡天衛星，是由美國航天局NASA發射的。最初的目標是用凌日現象研究銀河系中20萬顆恆星運轉以及他們的行星，當然研究的恆星中也包括一些δ Scuti星。
TESS – the Transiting Exoplanet Survey Satellite, launched by NASA, originally designed to explore the transiting exoplanets arounds more than 200 thousand stars in our galaxy, some of them being δ Scutis.
與上篇文章中提到的蓋亞任務類似，它能夠快速掃描天空並提供宇宙各個方向的數據，不像克卜勒只觀察浩瀚星空中的一小塊區域。話雖如此，來自開普勒的數據在這項研究中依然非常有用。
Operating in a similar way to the Gaia mission mentioned in the previous article, it is able to scan the sky quickly and provide data in all directions of the universe, unlike Kepler only focusing on one patch on the sky. That being said, the data from Kepler is still useful to be included in this research.
視頻為TESS的模擬運轉影像，它的四個攝像頭能全方位的快速掃描外太空，提供更多可以分析的數據
Simulation of TESS in space, it four cameras are able scan the space in all directions quickly, providing accurate data to analysis
從克卜勒和TESS的數據中，那組科學家們僅挑選出了擁有高頻率的振動的潛在的δ Scuti星（每天至少30次的震動）進行進一步的研究。選擇高頻是因為，一般在低頻的情況下才會出現有規律的震動，這意味著很多恆星在這一方面可能有著共同的特性。因此,為了保證只有δ Scuti星出現在這次研究中，高頻振動就變成了那個比較獨特的屬性，得以用於篩選。經過對整個樣本集的篩選，他們挑出了60個有規律間隔振動的候選樣本進行下一步的研究。
From the data of Kepler and TESS, the scientist singled out the potential δ Scuti stars with high frequencies of pulsations (30 or more pulsation cycles per day) for further research. The reason for choosing the high frequencies is because, regular pulsations are mainly found to happen at low frequencies, which means it would be a common property for multiple types of variable stars. So, to identify only the δ Scuti stars, a slightly more unique property had to be used. After filtering through the entire sample set, 60 candidates with regularly spaced modes of oscillations were chosen to be in next part of research.
視頻為名為HD31901的δ Scuti 星的模擬振動影像，模擬數據來TESS由雪梨的科學家們進行分析後得到的24小時振動模擬
Simulation of HD31901 - a δ Scuti star pulsating based on the star's brightness measured by TESS and analysed by scientists based in Sydney
像δ Scuti星這樣的恆星，它們會同一時間有不同頻率振動的發生，就像一個管弦樂隊同時演奏不同的樂器時候一樣。因此，各種振動模式將會在同一個數據中出現，當中隱藏著恆星各種信息。所以，這裡用到了échelle圖，顯示頻率的同時也顯示了定期間隔的振動模式（Δν）。
因為不同的振動模式來自恆星的不同區域，恆星內部的聲速值的變化會最後體現在我們監測到Δν的數據上。而聲速變化很重要是因為它包含的著恆星密度的信息。
Stars like δ Scutis have oscillations with different frequencies happening at the same time, like an orchestra playing different instruments at the same time, which creates a range of oscillation modes hidden in the star’s pulsation data we see. So, an échelle diagram is used to show the frequencies, and to determine the regular spacing of the modes of oscillation, Δν.
Because different modes of oscillation come from different parts of the star, the values for the sound-speed would vary inside the stars, shown to us with values of Δν. Here sound-speed variation is important because it carries information on the stellar density.
Échelle圖是用來表達恆星中多種頻率模式的圖，因為混合頻率常常會出現各種主序恆星中，所以這種圖能很好的幫助分析被研究恆星的振動頻率。圖中明確了每個不同的頻率和規律振動模式的關係。
Échelle diagrams are diagrams designed to show multiple modes of frequencies in stars, since mix modes is common appearing in evolved stars as well as stars in the main sequence. In it, the mode frequencies are plotted as a function of frequency of the regular mode oscillation.
圖為HD99506 的échelle圖，是60顆被挑選出來進行研究的δ Scuti 星之一
The échelle diagram of HD99506, which is a δ Scuti star studied by the Sydney tesm
下期揭曉
雪梨科研人員們
關於δ Scuti 星的突破性發現！
敬請期待！
Next, we reveal
the revolutionary discoveries of δ Scuti stars
from the team in Sydney!
長按關注 勿裡補說 公眾號，
了解物理點滴
圖片視頻來自TESS, MIT, Scitechdaily, NASA官網
部分信息參考來自的雪梨天文研究所團隊的 「Very regular high-frequency pulsation modes in young intermediate-mass stars"論文
Pictures and videos from official website of TESS, MIT, Scitechdaily, NASA.
*Parts are from 「Very regular high-frequency pulsation modes in young intermediate-mass stars" by the scientist team from Sydney Institute for Astronomy